A URL field is intended to store a single URL (not hyperlinks) in each cell. To add a URL field, click the + button located to the right of your existing fields, and select the URL field type. 

Now you can enter any single URL into the field. Clicking on a URL will first expand the field, after which you can click the URL to open it in the default web browser on your device. 

If you want to add a hyperlink instead of a full URL string, use our long text field with rich text formatting. Learn more about using this field in this article.