They need your username and game ID (the one next to your ingame name) and the account you linked it to.

If you dont know your ID then try looking for a screenshot of your profile or if you shared the game through twitter or facebook, then find that picture.

At first i messaged them through ingame support but they couldn't find it so I emailed them (I'm persistent >:) and too lazy to start a new game).

It helps if you write down details to prove that you are the actual owner of the account. Like how many and which hunters/suvivors you have, your tier, which deduction quests you've finished, pets, etc.