Open the Mycelium wallet app and go to the "Accounts" page (unlock if needed)
Click on the Account that says "contains private keys" then Click on the 3 little dots in the top right corner and select "sign message"
Enter your pin again to unlock the address list, click on the address you want to sign and this will open the page for you to sign the address.
Once you have signed the address copy the signature and text to your clipboard.
(remember to include the date and exact reason you are signing the address)