Here is how you can add a hyperlink or URL field to a layer in your Maps site:

1. Open the admin panel and search for the data source you want to add the hyperlink to in the Data Sources tab.
   
2. Click ADD in the "FIELDS" section next to "Add a new field".
   
   
3. Type a field name and select the field type. Choose wither Short, Medium, Long, or Custom Text. Be sure that the character length (in brackets) is long enough for your longest hyperlink. If you choose Custom Text, you can specify any character length between 1 and 1000 characters. Click Save.
   
   
4. Still in the admin, open the layer that uses this data source. 
   
5. Click edit in the Feature Info Pop-Up section.
   
   
6. Select List/Link from the drop down box at the top of the screen and add the new hyperlink field to the "LINK FIELD" section. Click Save.
   
7. Now you will need to create a new form to add hyperlinks to the existing data. Click the forms icon on the left and click Create New Form.
   
8. Select the layer that you just edited and click CHOOSE. Give the form a name and description and click Save. Make any form changes you want and click DONE.
   

Users with permissions can now click on a feature on the map and can add a hyperlink through the form.