You don’t need to claim any Ycash. The point is many people will have a copy of the ZEC private key used for signing messages for an address at the fork point so they could claim them either now or later.
It’s pretty straightforward to sign a message and see the post here from Andrew Miller Coinholder Petitions using t-Addresses. If you are using zcashd then the format is simply (from the RPC help signmessage - Zcash 4.5.1-1 RPC Docs)
zcash-cli signmessage "t14oHp2v54vfmdgQ3v3SNuQga8JKHTNi2a1" "my message"
With Trezor you can sign a message for an address in the wallet interface. I don’t think that’s currently possible with Ledger Live.
Of course, if you simply say publish this to your forum profile you’ll be revealing your balance.