<TelerikGrid Data=@GridData
             Height="500px" Pageable="true" PageSize=@PageSize>
    <GridToolBar>
        <GridCommandButton Command="Add" Icon="add">Add Employee</GridCommandButton>
    </GridToolBar>
    <GridColumns>
        <GridColumn Field=@nameof(SampleEmployee.Id) Editable="false" Width="70px" />
        <GridColumn Field=@nameof(SampleEmployee.Name) />
        <GridColumn Field=@nameof(SampleEmployee.LastName) Title="Surname" />
        <GridColumn Field=@nameof(SampleEmployee.AgeInYears) Title="Age" />
        <GridColumn Field=@nameof(SampleEmployee.HireDate) Title="Hire Date" />
        <GridCommandColumn Width="120px">
            <GridCommandButton OnClick="@((arg) => OpenEditWindow(arg.Item as SampleEmployee))">My custom edit</GridCommandButton>
        </GridCommandColumn>
    </GridColumns>
</TelerikGrid>

@if (ShowEdit == true)
{
    <TelerikWindow Class="demo-window" Width="500px" Height="300px" Centered="true" Visible="true">
        <WindowTitle>
            <strong>My title</strong>
        </WindowTitle>
        <WindowActions>
            <WindowAction Name="Minimize"></WindowAction>
            <WindowAction Name="Maximize"></WindowAction>
            <WindowAction Name="Close"></WindowAction>
        </WindowActions>
        <WindowContent>
            <div class="form-group ">
                <label class="control-label" for="productName">
                    Product Name
                </label>
                <input class="form-control" @bind="@selectedEmployee.Name" type="text" />
            </div>
            <div class="form-group ">
                <label class="control-label" for="productName">
                    Product Name
                </label>
                <input class="form-control" @bind="@selectedEmployee.LastName" type="text" />
            </div>
            <div class="form-group">
                <div>
                    <TelerikButton Icon="save" Class="k-primary" OnClick="@CustomEdit">Save</TelerikButton>
                </div>
            </div>
        </WindowContent>
    </TelerikWindow>
}

@code {
    public class SampleEmployee
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string LastName { get; set; }
        public int AgeInYears { get; set; }
        public DateTime HireDate { get; set; }
    };

    private SampleEmployee selectedEmployee { get; set; }
    private SampleEmployee originalEmployee { get; set; }

    private bool ShowEdit { get; set; } = false;

    private void OpenEditWindow(SampleEmployee employee)
    {
        originalEmployee = employee;
        selectedEmployee = new SampleEmployee()
        {
            Id = employee.Id,
            Name = employee.Name,
            LastName = employee.LastName,
            AgeInYears = employee.AgeInYears,
            HireDate = employee.HireDate
        };
        ShowEdit = !ShowEdit;

    }
    private void CustomEdit()
    {
        originalEmployee.Name = selectedEmployee.Name;
        originalEmployee.LastName = selectedEmployee.LastName;
    }

    int PageSize = 15;
    public List<SampleEmployee> GridData { get; set; }

    protected override void OnInitialized()
    {
        GridData = new List<SampleEmployee>();
        var rand = new Random();

        for (int i = 0; i < 100; i++)
        {
            GridData.Add(new SampleEmployee()
            {
                Id = i,
                Name = "Name " + i.ToString(),
                LastName = "Surname " + i.ToString(),
                AgeInYears = rand.Next(10, 80),
                HireDate = DateTime.Now.Date.AddDays(rand.Next(-20, 20))
            });
        }
    }
}
<style type="text/css">
    .demo-window {
        z-index: 44444;
    }
</style>