You can do that in two ways.

Your keyboard can store shortcut phrases. It’s under Settings -> Keyboard -> Text Replacent. For example, if I type ”aflink“ it expands to

“If you enjoyed reading this article, you can use my affiliate link to become a Medium member today and get unlimited access to everything at Medium! I’ll receive a portion of your monthly subscription fee at no additional cost to vou.“

Commercial apps like Textexpander can do much more.