I was asked this before. When I refused, they said it was mandatory. I put my number as 30% more. There is no way for them to confirm, I think this is an unfair

I was asked this before. When I refused, they said it was mandatory. I put my number as 30% more. There is no way for them to confirm, I think this is an unfair practice, so if they are forcing you to play the game, why you would be fair? I have got the job, as a junior, I was making same money as senior minus the benefits.