Open your prototype and, in the top-right corner, click the Share button.
In the share modal, click Link Settings, and then click More Options.
Click to toggle on Require user identification.
Note: If you are an Enterprise user, this setting will not appear if you have enabled the global setting, Require authentication for share links. When you require authentication, anyone who views the share link will need to be a member of your Enterprise organization and will need to sign in before viewing. To learn more, check out this article: Require authentication for share links