That would be a feature of the message app, not of the contact. I don't think the HTC message app supports this, but third party replacements do.
For example, with Chomp (the message app I'm currrently using) I just long-press on a contact's name in the conversation list and select "notifications" from the pop-up that appears, then choose the ringtone for that contact. It's similar with Textra, except that having selected the conversation you then use the 3 dot menu button to set the per-contact settings, and most other 3rd party apps also support this feature.