Its very simple to do bud, from the screenshots you shared, I see that you are using electrum wallet, I've never used electrum wallet before, so I can't possibly guide you with that, but if you will be willing to download mycelium wallet and imports the bitcoin wallet in question into mycelium, then the screenshot posted below should be easy to understand and follow..



Once you select your bitcoin wallet, you will see the "sign message" function at the top right corner, click it, if there are multiple wallet addresses under the same key, you will be asked to select the address you want to sign the massage from, select the bitcoin address in question, and the next page, type in what ever you want and click the "sign message" button beneath.