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I'm not exactly sure what you mean by sign a message in byteball?

The attestation bot is asking me prove ownership of my address by signing a message. How do you do that?

The request to sign message is just an alternative way of doing things. You don't actually have to do it though. It'll send you a link next to log into your steemit account.

Figured it out- I was using an older version.

I'm glad to hear that you solved the issue.

Did you try pressing the 3 dots on the bottom left corner and selecting insert my address?

I think I know what you're talking about now. You'll have to send a payment of 49000 bytes. Click on the blue link that says payment request 49000 bytes.