First, you'll have to insert a LOCAL SCRIPT to STARTERGUI. Name it anything you want, Its your choice. then you'll have to OPEN that LOCALSCRIPT and put this in:
game.StarterGui:SetCore('ChatWindowPosition', UDim2.new(POSITION HERE))
Heres how to relocate a frame and identify its position so you can use it for the script above: First, Insert a SCREENGUI to StarterGui and put a frame on it, DRAG IT anywhere you want (Hover it with your mouse, Hold left click, Move your mouse around to drag it) Then if you opened properties (Go to view tab on the tabs above your roblox studio and press PROPERTIES to open it if you haven't) there is a POSITION option in there, copy that and then replace POSITION HERE to the position you copied to the localscript.
Thats all I hope my demonstration was good! Thanks have a great day.