Sophos Mobile Control installing on a instance with non-default-port

hello

i try to install Mobile Control for evaluation beside my Sophos Endpoint protection bundle. 

i have a seperated MS SQL Database server with different instances on it. Therefore, port 1433 is allready used by an instance.

so i decidet to create a new sql database instance and assigned port 1438 to it (TCPIP)

but: i could not handle it to get connected to it during the installation.

i tried different formats:

\\servername\instanz:1438
\\servername:1438\instanz
\\servername,1438\instanz
\\servename\instanz,1438
\\servername:1438
\\servername,1438

but no success at all. 

im stucked here (see attachment)

Sophos_iLIKE