When ActiveTabId is null, the TabStrip selects the first tab automatically, but does not render its content.
<TelerikTabStrip @bind-ActiveTabId="@TabStripActiveTabId">
<TabStripTab Title="First" Id="t1">
First tab content.
</TabStripTab>
<TabStripTab Title="Second" Id="t2">
Second tab content.
</TabStripTab>
<TabStripTab Title="Third" Id="t3">
Third tab content.
</TabStripTab>
</TelerikTabStrip>
@code {
private string? TabStripActiveTabId { get; set; }
}