Basically they make you fill it, but I think the idea is that the company who'll hire you fill in the salary you claimed you had during the interview and the ba

Basically they make you fill it, but I think the idea is that the company who'll hire you fill in the salary you claimed you had during the interview and the background check company will check if you didn't lie. So you can just leave it out, if it's important they'll ask you and you can discuss why you're not confortable sharing it. Anyway it won't cost you the job.