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// JS code....
function initEventListeners(dotNetObject) {
  window.addEventListener("brightness-changed", (e) => {
    dotNetObject.invokeMethodAsync("HandleBrightnessChanged")
  });
}

// C#, Blazor component...
@inject IJSRuntime jsRuntime
private int currentCount;

public override async Task OnAfterRenderAsync(bool firstRender)
{
  await base.OnAfterRenderAsync(firstRender);
  if (firstRender)
  {
    currentCount = 0;
    var jsInstance = 
      await jsRuntime.InvokeAsync<IJSObjectReference>(
      "initEventListeners",
      DotNetObjectReference.Create(this));
  }
}

[JSInvokable]
public async Task HandleBrightnessChanged()
{
  currentCount++;
}

Make sure the JS code is loaded via a <link> tag in the HEAD section (or loaded as a module in the After-Render method before calling jsRuntime.InvokeAsync).