How do I intercept routing in Blazor before it navigates?

Create the class file [RouteData.cs].

 …………………. ..
 namespace BlazorApp.Data
 {
     public class RouteData
     {
         public string Textfield { get; set; }
     }
 }

To register the RouteData class as a scoped service in Program.cs, you can use the following code snippet.
[Program.cs]

builder.Services.AddScoped<BlazorServerApp.Data.RouteData>();

Add the following code to the [Index.razor] page.

 @page "/" 

 @inject Data.RouteData RouteData
<p>Type the text below to allow Home page intercepting</p>
<textarea @bind="RouteData.Textfield"></textarea>

Check the condition and allow the navigation process in the [NavMenu.razor] page as shown.

@inject NavigationManager NavigationManager
@inject {Your App name}.Data.RouteData RouteData
<div class="@NavMenuCssClass" @onclick="ToggleNavMenu">
     <ul class="nav flex-column">
         <li class="nav-item px-3">
             <NavLink class="nav-link" @onclick="Navigate" Match="NavLinkMatch.All">
                 <span class="oi oi-home" aria-hidden="true"></span> Home
             </NavLink>
         </li>
                   ……………………. . .
     </ul>
 </div>
 @code {
     private bool collapseNavMenu = true;
     private string NavMenuCssClass => collapseNavMenu ? "collapse" : null;
     private void ToggleNavMenu()
     {
         collapseNavMenu = !collapseNavMenu;
     }
            private void Navigate()
     {
         if (RouteData.Textfield == null)
         NavigationManager.NavigateTo("");
     }
 ………………… . . .

Home page routing is intercepted when you type text in the text area. When the text area value is null, the app navigates to the home page.

View Sample in GitHub