If you have already sent a payment, Then you can sign the original BTC address you sent the payment from as a way for proving that you were the one who actually sent the payment. You can read this article^[1] it has a bunch of guides explaining how to sign a BTC address with different wallets. You can basically say in your signature that this address belongs user XYZ that way the person who received the BTCs can verify that it was indeed you who sent these BTCs. Also, Which wallet are you using? If you are using something like Electrum then you can simply paste the URL in electrum and it will automatically detect all the details just like @khaled0111 mentioned.

---

Source:
[1] https://coinguides.org/sign-verify-bitcoin-address/