Want to run T-SQL Job with as specific but I can't select the user

Heyas,

As the above title: Want to run T-SQL Job with as specific but I can't select the user.

Attached is a screenshot of what I mean.

I have created SQL user SQL_RG_MP_User and given the sysadmin permission but they don't appear on the list.

Any assistance is appreciated.

Thank you.
Capture.PNG