How can I add a contact to Favorites?

If you mean pin them so their icon appears at the top of the list screen in the Messages app, hold your finger on the thread and when the menu appears choose Pin.

If you mean to the Favorites list in the Phone app open Contacts, tap on the contact, scroll down, and tap Add to Favorites.

You can add VIP contacts to your Favorites list for quick dialing and imessage

Select a contact, scroll down, then tap Add to Favorites,

select the contact you want to add to Favorites and choose Message, Call, Video, or Mail.

or Follow this link below for more guide

Use Contacts from the Phone app on iPhone - Apple Support (uk)