1. Place the component

<HUIReconnectModal />

in the body of your App.razor page. It should be right above your blazor js import.

2. Set autostart to false

<script src="_framework/blazor.web.js" autostart="false"></script>

3. Import boot js

<script src="@Assets["_content/Hedin.UI/js/boot-server.js"]"></script>

If you are running blazor webasm then you need to implement your own boot.js script. Microsoft has an example here

The same way you could of course implement your own blazor server reconnect logic too. Replace boot-server.js by your own javascript.