@Richard-Miami ...
I don't know of a way to do this, unfortunately. Are you wanting to only disable this textbox when you are using an external tool for your submission type? I would think that if you were somehow able to disable this, it wouldn't disable it just for external submission types...but for all assignments...and it might not be just for your course(s)...but for all other courses at your school that are within the same Canvas account/sub-account structure. So, there may be much larger ramifications for doing something like this that would need to be considered by your school's Canvas administrator(s) or someone from their Online Learning / eLearning / Distance Education team. My $0.02 ... for what it's worth.
