I haven't used the nuget yet,
try making a method in game1 that sets IsFullscreen to true and call ApplyChanges().
If thats not enough you might need to also set the prefered back buffer to the current adapter display mode width height or DisplayMode's, before calling apply changes or isfullscreen = true;
If it has to be a toggle your probabaly going to have to cheat with some bool switching,
If(IsFullscreen){ IsFullscreen = false;}
else( IsFullscreen = true;}
You will have to time it if its linked to a button, so it doesn't double fire or fire like 10x in a row.
That is if it works at all.
Togglefullscreen was broke a long time ago im not sure it was ever fixed on the regular gl version either. If i remember IsFullScreen had some quirks too about getting called to early and you might have to initially do it at the end of load content to start it that way. I don't about nuget at all but.
That's basically how i manually worked around it on the regular gl version. I just made my own static call which required static references to the window and graphics those references also must be re-assigned at the bottom of load content (not initialize) in case of a device reset at least graphics does, that's if it works and or you intend to call it from outside the class.
