Hi there @PamelaDooley !
Thanks for asking on the Instructure Community!
Unfortunately, Canvas does not provide you with a direct way for text to automatically appear in inbox messages (except for signatures, if enabled by your institution) or announcements. To get around this, I recommend creating a page that you can just copy and paste the text out of, so you can paste it right into your message without having to manually type it out.
If you'd like to, however, since this seems like a cool feature to submit as a feature idea, I'd encourage you to submit this as an official feature idea for the product team to review for possible future implementation into Canvas! You can view more information about submitting feature ideas here: https://community.canvaslms.com/t5/Instructure-Community-Guide/How-do-I-create-a-new-idea-in-the-Ins...
I hope this helps, all the best!
Noah