I can find no information on how to insert text after the last character of a field or document. I'm sure I can copy the document text into a variable, append text to that variable, select the document text, and paste the modified text into the document. I'm hoping for something much more simple.
It depends on the situation, but I usually use the standard macOS shortcut ⌘↓ to go to the very bottom of the document or field.
Just put that keystroke in a KM Type a Keystroke action.
Depending on what you’re doing, you should also read about the Append action. I use it frequently to add text at the ends of various text files.