I need more detail on how to do this? Can you create a bookmark without being on a page - and making a bookmark of that page? Mystified. I wish roll20 had an "In-person" setting...

Hide the UI Bookmarklet

If you add a TV/Monitor to your game, you may want to hide the User Interface on the player's screen. You can create a bookmarklet (javascript used to temporarily alter a web page's appearance) to hide the UI.

   example; 
   Create a bookmark in your browser and paste the code below into the URL field. To hide the Roll20 UI, simply click on the bookmark once the Roll20 editor has loaded. Refreshing the editor will restore the UI. 

javascript:(function(){$("#zoomslider").css("display","none");$("#floatingtoolbar").css("display","none");$("#sidebarcontrol").css("display","none").click();$("#playerzone").css("display","none");$("#initiativewindow").parent().css("left","-10000px");$("#page-toolbar").css("display","none");$("body").addClass("sidebarhidden");$(".dark-mode-switch").css("display", "none")})();

   Adjust the parameters as needed. For example you could remove elements from the bookmarklet code above for sections of the UI you may not want hidden.