Open the messaging app and start a new conversation (or open an existing one). Then, tap the "+" to the left of the text entry box, then the menu button on the far right (3 vertically-stacked dots). On my T-Mobile GS9, "Scheduled message" is the first option under that menu.
That's a cool feature. I was looking through Textra and I didn't know think it had that option. I just found it, looks different, a little clock symbol instead