in the dialog, where you want the window to appear, make it as a button, including the size. So if you wanted a pretty big window, it would originally be a pretty big button.
from there, you will need both mdx.dll and dialog.mdx
dll $mdx SetControlMDX $dname 5 Window > $dialogmdx
window -h @ $+ %gdnick -1 -1 700 300
did -a $dname 5 grab $window(@ $+ %gdnick $+ ).hwnd @ $+ %gdnick
the above quote is from my whisper dialog that i have in GD4.x
basically you need to change 5 to the id you want. $mdx = mdx.dll and $dialogmdx = dialog.mdx
@ $+ %gdnick would be the window you want, so in this case you would have it as say @test intest of @ $+ %gdnick
then you would grab that window, and put it over the top. ofcourse it does take some playing around to get it right, but the steps i have shown you here should get you there.