Exporting a Link to Excel

I am trying to export a link to an Excel file and I can't get it active.

Please see the screen captures below:

This shows the code that puts the link into the MySQL file"Letter03"

This shows the API that generates to Excel File.

This shows what comes out in the Excel File.

The text beside the error is not active. What code should I have in the first screen capture that would result in an active link in the Excel File.

Any help would be appreciated.