There are two codes fields for frequent flyer programs in an United ticket record, they are FQTV for mileage credit and FQTS for status recognition

Usually the request is to use UA status to get E+ and have the mileage credit go elsewhere, your request seems to be the the reverse of that. Note this may effect your ability to get E+.

Most agents are not familiar with these codes so you may need to be patience. Changing after the flight can also be difficult.

Search on either FQTV or FQTS for more details, one such thread is
2 *A programs on a United flight?

Another possibility would be to use your US number for check-in but once you reach the gate ask the GA to chnage it back to the UA number.