my yorkie does not like the carrier either. We just walk him.
But what you can do is leave the bag open near his/her crate or play area with teats in it. Put his her favioret toy in side it as well. do not encurge or make an issue of the bag. just keep droping treats in it ... while they are looking.
Eventually they will enter the bag then give hugs and kisses and leave him/her to play. I did this for two day with chelsey our maltease. Then I took her out to the vet on day three. Now when I take the bag out of the closet she knows we are going outside. She comes running. You thing she would be afraid of the vet but she likes it there. More treats for her.
