@inject IJSRuntime _js

<TelerikButton OnClick="@PromptWithText">Workaround</TelerikButton>

@code{
    [CascadingParameter]
    public DialogFactory Dialogs { get; set; }

    async Task PromptWithText()
    {
        // the workaround - invoke a script that will set the input value with a delay
        await _js.InvokeVoidAsync("setPromptDefaultText", "predefined prompt value");

        // do the prompts as usual
        string prompt = await Dialogs.PromptAsync("Enter something");
        Console.WriteLine(prompt);
    }
}

@* move this script to a proper location, this is a hack 
    to put in the blazor component so the workaround is easy to copy and run *@
<script suppress-error="BL9992">
    function setPromptDefaultText(defText) {
        setTimeout(function(){
            var promptInput = document.querySelector(".k-prompt-container .k-textbox .k-input-inner");
            if(promptInput) {
                promptInput.value = defText;
                promptInput.dispatchEvent(new Event('input', {bubbles: true} ));
            }
        }, 50); // a timeout so the prompt can render first
    }
</script>

    body {
        display:flex;
    }

<TelerikWindow Modal="true" @bind-Visible="@isModalVisible">
    <WindowTitle>
        <strong>The Title</strong>
    </WindowTitle>
    <WindowContent>
        I am modal so the page behind me is not available to the user.
        <br />
        <TelerikButton OnClick="@( _ => isSecondModalVisible = true )">Open the SECOND window</TelerikButton>
        <TelerikButton OnClick="@ShowConfirmWithTitle">Show Confirm</TelerikButton>
    </WindowContent>
    <WindowActions>
        <WindowAction Name="Minimize" />
        <WindowAction Name="Maximize" />
        <WindowAction Name="Close" />
    </WindowActions>
</TelerikWindow>

<TelerikWindow Modal="true" @bind-Visible="@isSecondModalVisible">
    <WindowTitle>
        <strong>The SECOND</strong>
    </WindowTitle>
    <WindowContent>
        Second modal!
    </WindowContent>
    <WindowActions>
        <WindowAction Name="Minimize" />
        <WindowAction Name="Maximize" />
        <WindowAction Name="Close" />
    </WindowActions>
</TelerikWindow>

<TelerikButton OnClick="@( _ => isModalVisible = true )">Open the window</TelerikButton>

@code{
    [CascadingParameter]
    public DialogFactory Dialogs { get; set; }
    bool isModalVisible { get; set; }
    bool isSecondModalVisible { get; set; }
    async Task ShowConfirmWithTitle()
    {
        bool isConfirmed = await Dialogs.ConfirmAsync("Are you sure?", "Confirmation!");

        Console.WriteLine($"The user is sure: {isConfirmed}.");
    }
}

@inject IJSRuntime _js

@* move this script to a proper place in your project and remove the 
    suppress-error hack that lets it stay in the component - it is here for brevity only *@
<script suppress-error="BL9992">
    function focusCancelDialogButton() {
        setTimeout(function() {
            var cancelButtons = document.querySelectorAll(".k-dialog .k-dialog-buttongroup .k-button");
            if(cancelButtons && cancelButtons.length > 1) {
                cancelButtons[1].focus();
            }
        }, 100);
    }
</script>

<TelerikButton OnClick="@ShowConfirm">Show Confirm</TelerikButton>

@code {
    [CascadingParameter]
    public DialogFactory Dialogs { get; set; }

    public async Task ShowConfirm()
    {
        await _js.InvokeVoidAsync("focusCancelDialogButton");
        bool isConfirmed = await Dialogs.ConfirmAsync("Are you sure?");

        if (isConfirmed)
        {
            Console.WriteLine("The user is sure, continue.");
        }
        else
        {
            Console.WriteLine("The user changed their mind");
        }
    }
}

Microsoft.JSInterop.JSException: Cannot read properties of null (reading 'addEventListener')
TypeError: Cannot read properties of null (reading 'addEventListener')

<TelerikDialog @bind-Visible="@Visible"
               Title="@Title">
    <DialogContent>
        <TelerikGrid Data=@GridData Pageable="true" Height="400px" Width="700px">
            <GridToolBar>
                <span class="k-toolbar-spacer"></span> @* add this spacer to keep the searchbox on the right *@
                <div onkeydown="event.stopPropagation()">
                    <GridSearchBox />
                </div>
            </GridToolBar>
            <GridColumns>
                <GridColumn Field="@(nameof(Employee.EmployeeId))" />
                <GridColumn Field=@nameof(Employee.Name) />
                <GridColumn Field=@nameof(Employee.Team) Title="Team" />
                <GridColumn Field=@nameof(Employee.IsOnLeave) Title="On Vacation" />
            </GridColumns>
        </TelerikGrid>
    </DialogContent>
</TelerikDialog>

@code {
    private bool Visible { get; set; } = true;
    private string Title { get; set; } = "Software Update";

    public List<Employee> GridData { get; set; }

    protected override void OnInitialized()
    {
        GridData = new List<Employee>();
        var rand = new Random();
        for (int i = 0; i < 15; i++)
        {
            GridData.Add(new Employee()
                {
                    EmployeeId = i,
                    Name = "Employee " + i.ToString(),
                    Team = "Team " + i % 3,
                    IsOnLeave = i % 2 == 0
                });
        }
    }

    public class Employee
    {
        public int EmployeeId { get; set; }
        public string Name { get; set; }
        public string Team { get; set; }
        public bool IsOnLeave { get; set; }
    }
}