To link your account and not have to Download 7GB :
Go to your Steam Folder
Normally C:\Program Files (x86)\Steam\
steamapps\common and then make folder called War Thunder
Note there is a SPACE between War & Thunder
and take the files from the folder in C:\Program Files (x86)\War Thunder (Your standard War Thunder Game Folder) and drag and drop (Or Copy) all the files into newly created folder in steamapp/common
then go to Steam Store click to PLAY the game which will ask to install, it will now install files but will install about 900mb of files instead of 7GB!
Of course you can play without Steam but might miss out on exclusive pack deals and Sales.
Once through Steam log in with your normal WT username & password