FYI since this thread has high SEO (high in google search) for any samsung phone.
S10 / S10+ the version of OS + newer Samsung tweeks changed the above advice:
In Messages, type message
- Click Left arrow to left of text box
- Click the 4 squares (right in the list)
-Click Schedule message (geeze so many clicks!)
But a few new (to me) options; send location., a subject, many other options now.
The hair in the back of my neck stood up when I was playing with my new note 10+ and a feature like many hear depend on was missing. Thankfully found. WHEW.