I had a beard transplant 1 year ago and have gotten the full results of it. It's patchy in some areas and want to use minoxidil?

Minoxidil will not make your hair transplant result look any better.  If you are not pleased with your results it would be best to contact your doctor for a follow up.

I think minoxidil usage nor  makes a benefit for your facial transplanted hairs neither causes thinning or bad effect on beard hair. You need a new revision surgery for achievement of desired density .
You may plan  again a hair transplant surgery from scalp skin follicular units to beard area with FUE to have more facial hair.

Minoxidil can be used in the hope that it will grow enough hair in-between your transplanted hairs to fill in the beard. If it does not work, then another transplant may be indicated