This setting makes it so that students must sign in to get access to an assignment, even if they are joining from a quick link. A normal link gives students the option to sign in or enter a name and do the work without associating with an account. If students do not sign in, their work is not saved for later and they are not able to view reports aside from right after they finish the assignment. Use is setting if your students have accounts but you want to post a link (i.e. post to Google Classroom or Schoology stream).
1. Open the Assignment Editor by going to Assignments, My Assignments on the navigation bar to the left, then clicking on the pencil to the far right of the assignment name.

2. When in the Assignment Editor, click on the Settings Gear at the top right, then Open all Settings from the dropdown.
3. Basic Settings will be selected by default from the tabs on the left. The first option under this tab is Require Sign in for Quick Links - check this and click Save Changes. Now, the assignment will require students to login.