Congrats on making your first post here in the Community. I appreciate you coming here with your question about adding a default text on a check memo line.
At this time, this option isn't available to your QuickBooks Online account. You'll need to manually enter the text each time when creating a check.
You can send our Product Developers some feedback so they can consider this in the near future. Also, check out our Blog site to keep up with what's new in the system.
I'm only a post away if you have any other questions. Have a wonderful day!