Hi,
It looks like this is a defect in the vCD API that will be fixed in the next release. The current workaround is to modify the GuestCustomizationSection of the VM after the composeVApp operation is finished.
From your screenshot, it looks like your vApp/VM is started/deployed, that's why you cannot change the Guest OS Customization settings.
You can try the following:
1) pass 'false' to the 'powerOn' and 'deploy' attributes in <ComposeVAppParams>
2) once the Compose vApp operation succeeds, change the Guest OS Customization settings of the VM
3) power on the vApp/VM
Regards,
Todor Todorov