I've submitted psets 1 to 8 through submit50 on the cs50 ide. Now I want to submit pset0 (the Scratch Project). How can I do so?. I'm using the edx cs50 ide of the 2017 course.
Consult the Problem Set 0 specification. In particular, the section at the bottom titled "How to Submit" will answer your question.
At the bottom of this page there are a couple of paragraphs telling you to share the project, but then I also believe that you need to submit the form under the first item on "How to Submit". This is because the form asks you to input the link to your shared project among other questions.