OP, you're probably referring to having staked an address on this forum, like
this thread in Meta. This involves signing from a Bitcoin address you own, and having someone there verify the signed message and quoting it, so that it can't be deleted from records.
It also means you should have done it
before your account was hacked, so if you're asking how to sign a message, then you probably haven't done so before... in which case it's pretty pointless to do so now. You could try signing from a Bitcoin address associated with your old account, but it doesn't prove that you owned the address while it was associated with your old account.
Protect your account by staking an address!