pkexec authentication for non-"root" sudoer

Ok so perhaps the basis of this question comes from my negligent understanding of a super user but here goes: I have a user that is on the sudoer list, call them user1. I can switch to this user and run commands with sudo using the password for user1. I do not, however, know the password for the user root (who is inherently on the sudoer list). I would like to be able to run pkexec in a way that requests the authentication credentials for user1 (not root). MY understanding is that the --user flag for pkexec is to run the command as that user (not request auth credentials for that user). So, how can I have pkexec request authentication credentials for user1?