Is there a way to add a diamond shape in my data model so I can type one word in it to inform users what action is happening between one table and another?

In Vertabelo, we have implemented the crow's foot ERD notification, and unfortunately this kind of notification doesn't use diamond shapes.
But you can use notes to add some comments to your diagram.
You can place a note above a reference line or next to the table, and type anything you want within this note. For example, this way you can describe interactions between two elements of your diagram.