@syoung123 If you pay electronically now the payment becomes part of your tax account based on your Social Security number. The IRS has access access to your tax account when they start to process the amended tax return so whomever is processing the return will know that payment was already made.
If you do mail a check with the return you can write in your phone number either to the right of your name or directly below your address. Anywhere in the body of the check or at the top you can write "Amended Return..."
I am always personally hesitant to writing a SS number on a check but you can if you wish and write it in the Memo section of the check.
