How do I run a diagnostic test on my iPhone XS

You use the Contact Support link at the top of this page, or download the Apple Support app to the phone. Then contact an Apple tech who can run a remote diagnostic on your phone.

Either call Apple who can remote in to your iPhone or go to your closest Apple Store Genius Bar and they can run diagnostics. You yourself cannot. If you didn't add AppleCare to your phone when you bought it, you had 90 days of free phone support. If you didn't, there will be a charge, but going to your Apple Store will be free.