Signing a wallet:
https://coinguides.org/sign-verify-bitcoin-address/Almost all Bitcoinwallets offer that feature.
You just write todays date and your username and sign it with an btcaddress that has a few BTC on, and post the result here
We can verify signature and need the address, custom message as well as the signature.
So we can see you have access to that wallet.
And there should be a reasonable amount of BTC in it, thats why we ask for it.
If you cannot show that, you dont hold coins, or at least nobody believes you that you hold 'em...