It seems you are playing with the SQL Authentication for the first time ;) .
SQL Authentication is my favorite. It reduces lot of your code work regarding authentication and authorization.
The solution to your problem is providing explicit access to the webpage you want to show without authentication. Just add below lines in your
Web.Config and you are ready to go:
Under the <configuration> tag enter below lines-
<location path="File Path">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>