how do I add a customized text tone to an iphone 6s?

Settings > Sounds > Text Tone

Are you asking how to change the default text tone or the text tone of a contact? Or are you asking how to add a text tone to your phone?

sberman answers how to change the default tone. If you want to change the tone for an individual, go to that person's entry in Contacts, tap "Edit" in the upper right corner, go down to Text Tone and change it to what you want, then tap "Save" in the upper right corner.

If you want to add your own text town to the phone, well, it's certainly possible to make your own, or acquire them from any number of sources and sync them to your phone using iTunes. An internet search should provide instructions. However, it is, in my opinion, a frustrating exercise. I buy them from iTunes by going to Settings>Sounds>Text Tone then typing the link in the upper right to buy more from the iTunes Store.