Press the [Windows] Key + [R] at the same time
Copy the following in to the text box and press ok
%localappdata%\swtor\swtor\settings
Open the client_settings.ini file
Check NativeHeight, NativeWidth, Height and Width ( all 4 ) match your monitor resolution.
Native height and native width is important it is the default resolution if trying to play a lower resolution , setting the height and width is the resolution your wanting for the game itself
from there you can add below for fullscreen window mode
Fullscreen = false
D3DFullScreen = true
