Open the Shortcuts app
Tap Automation
Tap New Automation or the plus button
Select Time of Day
Choose the date and time
Tap Next
Tap Add Action
Search for and select Send Message
Enter the recipient and message
Tap Next
Turn off Ask Before Running if available
Tap Done
Use a third-party scheduling app if your iPhone version does not support message automation
Set the scheduled time in the app
Confirm the message and recipient
Save the scheduled text