You type the short phrase or letters you've assigned to the short cut and your phrase should appear as an option to select (or, you can hit the space key to auto select it).
For example, I've made a shortcut for my name to be "ma." When I type those letters (and only those letters), I get my name. I can either press the pop-up to accept it or press the space key. If I continue typing a word, such as marry, then the option to use my shortcut disappears.
This works anywhere on the iPhone, to include the Messages app...
Marilyn