(1) Go to the "People" tab [1]
(2) Click the "+" Icon on the top right [2]
(3) Add the phone number you want to message. [3]
[Reference Images]
[1] https://scontent.fsnc1-1.fna.fbcdn.net/hphotos-xfa1/t39.2229...
[2] https://scontent.fsnc1-1.fna.fbcdn.net/hphotos-xap1/v/t1.0-9...
[3] https://scontent.fsnc1-1.fna.fbcdn.net/hphotos-xft1/v/t1.0-9...