The original text provided by the one and only Chris Hofer is as follows:
"When creating an Assignment in a course that uses the Submission Type of External Tool (Example: Turnitin LTI app), there isn't a way to add a Canvas rubric after the Assignment settings are saved. The work-around for this is to create an Assignment the normal way (and not selecting External Tool as the Submission Type), and then you can add your rubric. Next, go back into the editing screen of the Assignment, and change the Submission Type to External Tool. You can then access your rubric via the SpeedGrader for grading students. It would be nice to be able to also have an "Add Rubric" button on the screen for those assignments submitted through an External Tool."
I don't think I could say it better. We really need this to smooth out the not-so-smooth workflow of adding a rubric to a Turnitin assignment. With how heavily many institutions use Turnitin, this is not so much a feature request as it is a "must."