How do I use a text box to create a working link for the signer?

Change the Data Label so that it begins with '#HREF' (ex. #HREF TextField1).

Copy the complete url into the field's Tooltip.

You can use whatever text you want for the field value, just make sure the field is marked as Read-Only. You can also change the text color to Bright Blue.

When the signer enters the form and clicks on the link, a pop up will warn that that the external link will open a new tab/window. Click continue to open the link.