Reserve Management Port for Cluster member

Hi,

I have Fortiweb in HA with Mgmt1 interface used for management. I want to use this interface for Reserve Management Port for Cluster member. Once I tried I am not able to do it, there is no option to select Mgmt1 port. Even I tried this on VM in LAB with Port1 port as a management Port and now trying to configure it Reserve Management Port for Cluster member, same not getting option to select Port1.

Is there any limitation to not able to use Mgmt1 & Port1 (For VM)?

Regards,

Atif.