I would like to share answer keys with students who have submitted work, I can choose the option for students to required a link in order to access but i am not sure how or where the link comes form.
What you could do is put those answer keys in a Module and set the Module to release when certain criteria has been met, like a certain score on an assignment.
Here is the guide for doing this:
How do I add requirements to a module?
Only the students that have met the requirements can access the content in that Module.
You may, depending on how serious the assignment is, want to wait until the time to submit it has expired. If you share it with students who have submitted the assignment, they can easily share it with someone else and that bypasses any restrictions the modules may place on things.
You can also schedule the availability of the file itself, so that it doesn't become available until a certain time, no matter what progress students have made.
Here is a lesson from the Canvas Instructor Guide with more information: How do I restrict files and folders to students in Canvas?
As to your original question, the link comes from where you select course content to include. When selecting course content to add, you go to Files instead of pages, announcements, discussions, quizzes, etc.
Retrieving data ...