Originally what you had to do was go to the server hub and do /link which allowed your account to link to your account on the forums.
From this what would happen is with the rank you have on the server (VIP, VIP+, MVP) it would give you the donator title on the forums.
I have tried to get it myself I did /link in the server hub but the command as been removed so really to sum it up. There is no way currently to get the donator title. ;-;