I have written on the website about TextExpander, a utility that looks for pre-defined text abbreviations and automatically inserts a block of text assigned to the abbreviation. However, if you feel an application such as this is either overkill for your needs or you are unable to cover the cost of another subscription there is a solution, use the one that is built into macOS.
Access System Preferences by clicking the grey cog (img.1) icon then choose the keyboard icon (img.2).
Once in the keyboard system preferences pane click on the text tab along the top of the window (img.3).
You will probably see an entry already in 'Replace' and 'With' columns which will be omw and On my way! This means that while in applications on your Mac if the characters omw are typed they will be replaced with the phrase On my way!
To enter your own text replacement click the '+' (img.4) enter the abbreviation you wish to use in the left column and then tab and enter what you want the abbreviation to be replaced with in the right column and then press return. In my example I have used the abbreviation mqem to be replaced with email@example.com (the mac quad email address) (img.5).
Now whenever mqem is typed it will be replaced with firstname.lastname@example.org saving lots of time when I need to type the email address.
This does not have the advanced features of TextExpander but is sufficient for basic text replacement.