In FileMaker you can place a web viewer on your layout. But only when you actually load an URL into the web viewer, FileMaker will create the control and place it on the window. To solve this, we often load "about:blank" to get a blank web viewer.
To verify, here a FileMaker window in 3D view in debugger, where you see the view hierarchy without a web viewer:
And here after loading about:blank with the web viewer:
As you see FMWKWebView is now present and MBS Plugin can find it with WebView.FindByName and other functions. Alternatively you can use WebView.Create or WebView.CreateOnPopover to add a web view via plugin functions.