1. In onMessage, select Content > Uploads (Misc).
  2. Select Add in the upper right-hand corner.
  3. Choose the file type from the menu.
  4. Enter a description for the file.
  5. Select Choose File and browse for the file on your computer.
  6. Select Save & Exit.
  7. Click on the blue File Name link, and copy the URL of the file
  8. Insert this URL where you need the Link to display
An example is schoolname.myschoolapp.com/ftpimages/236/misc/misc_70321.pdf