How to fill a butt dimple/indent? With little BMI

Hello dear!!
Thanks for the question and provided information as well.
Looks like you could benefit from a fat transfer procedure, but unfortunately, without pictures, it is very difficult to give you a proper answer. 
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

I have had a lot of success using Sculptra for buttock augmentation. I have also used Sculptra for patients with buttock indentations from steroid injections or uneven fat grafting.  The nice thing about Sculptra is that it forms collagen, so it is a long lasting procedure.

Thank you for your question. Unfortunately, with no pictures to analyze the defect, I can only offer proposed solutions. I would say that if the defect is of small-moderate size, your best option would be fat grafting to the area augment the dimple and change the contour. I realize that you are thin and have a low BMI, but with most patients, there are areas where fat can be removed for use in fat grafting, especially if the area does not require a large amount of correction. Also know that with fat grafting, sometimes it takes multiple procedures to get the desired look as the take of the fat is somewhat unpredictable. Have a consultation with a local qualified plastic surgeon and let them examine you. After that, you can discuss your goals and expectations and go over the options that might be available to you. Best of luck!