Message signing is useful whenever you want to prove ownership over the keys associated with a certain address. Some exchanges require you to do such operation before enabling you to withdraw from the exchange onto such address. They will provide you a message template including some code or ID that will allow them to be compliant with their local regulation regime.
Note: Message signing is only available for singlesig accounts.
1. First, open the relevant wallet and navigate to the Transact tab via the navigation bar at the bottom of the screen. Then, select Receive.
2. Your Blockstream App should now display an unused Bitcoin receiving address by default. Tap the icon with the three dots in the top-right corner to bring up More Options. In that list of options, select List of Addresses.
Note: You will need to switch the Asset to Liquid Bitcoin if you'd like to prove ownership over a Liquid address rather than a traditional, on-chain Bitcoin address.
3. You will then be presented with a list of addresses associated with your wallet. Find the address you'd like to use and tap the signature icon on the far-right side of it.
4. Finally, you can type or paste the message you'd like to sign with your chosen address and tap Sign message. You will then be presented with the signature for the message you signed with the chosen address, and you can use copy and paste to send that signature anywhere you'd like.