As far as I know, dress designers in wedding shops are much more advanced than ordinary salespeople! Generally speaking, you should have a good ability to match clothes and make up the whole wedding. Relatively speaking, you should have certain professional ability. You study fashion design. Personally, I think you should have no problem doing this position. Of course, the store will give you some corresponding professional pre-job training. Don't worry about this!
However, it should be noted that some shops will give them the title of dress designer in order to facilitate the sales of shopping guides, which is actually an ordinary sales shopping guide. If so, you need to have a certain sales ability!
I feel that you want to be the kind of fashion designer described in the first description, so be sure to ask clearly when applying!
I hope I can help you!