One plus 3T - Manual (Page 18)

18 | What’s in the Box Device OxygenOS Safety Information
Messenger
To send an SMS, launch the Messenger application on your home
screen or application drawer. Enter the name or phone number in the
field or select someone from your contacts. Type your message, then
tap
For an MMS, in the Messenger application, select the media you want
to add . Type your message, then tap