I think to make it full screen, you’ll have to do 3 things in function that makes it go fullscreen:
1. Change frmMain to borderless form (set BorderStyle to 0 - none)
2. Move it it 0,0 (frmMain.Left=0, frmMain.Top=0) with remembering in some variable old values.
3. Change resolution to 1024x768 (closest one to Game’s main window size) with saving current resolution, both can be done via WinAPI
For going back from fullscreen:
1. Restore resolution from previously saved value
2. Restore window’s position
3. Change frmMain’s BorderStyle to 1.