) goes on saying that If you are connecting to a web application that uses forms-based authentication, then you must specify a preferred URL that uses Windows authentication.
To specify a preferred URL:
From the Connection Manager page, select the connection.
In the bottom-right corner of the window, select the preferred map from the list of alternate access maps.
Click Save.
Extend the FBA Web Application to a new Web Application that uses Windows Authentication / NTLM