I purchase soft soap in the large containers at Costco. But they only come unscented. I like the melon-flavored soft soap that is available in the small bottles that shows a watermelon on the front.

Is there a melon flavoring that I can add to my large container? What is it called, where would I purchase it and how much would I add?

Thank you for any advice.

Check the list linked below. These fragrances are supposed to all be safe for use with soaps, etc., but just to be sure, find one that says "soap safe," "safe for soaps and lotions," Safe for soaps & bath/body products," "skin safe" or something similar.

As to how much to add, it depends on the quality of the fragrance you buy, and how much soap you are adding it to.
I'd start by putting a portion of your large container of soap into a smaller container, then adding fragrance a bit at a time until you get it the strength you like. Just be sure to keep track of how much you have added so you can duplicate it later.

And look for instructions/suggestions on the site that you buy the fragrance from on how much to add. The reliable sites have lots of good information.