Making Button Links
You can convert any link created in the rich text editor into a button link by applying the button class from the to it.
First, create your SItecore link or External link in the rich text editor. Right-click on the link and then select Properties....
The Link Properties modal will open. Click on the CSS Class drop-down, locate the btn, .btn-primary class and select it.
Click OK, and the class will be applied to your link and change it into a button.
Accept the changes in the rich text editor, and be sure to save your page.