APIM developer portal test console does not show gateway custom domains

I registered a custom domain for an APIM gateway. In the developer portal's test console, only the custom domain for the gateway is shown and automatically used.

Details: The APIM instance is deployed in internal (vnet) mode. The default gateway domain is only accessible within the vnet (and on-premise network). The custom gateway domain is used for public access and routed through an on-premise firewall. The developer portal is only accessible from the vnet (and on-premise network) and should therefore use the default gateway domain. Like this, we can use the developer portal in a protected vnet environment and protect the public custom gateway domain with more elaborate measures.

I would expect at least to be able to select which gateway domain should be used for the request (if not even choose a default domain other than the default gateway domain)

I registered a custom domain for an APIM gateway. In the developer portal's test console, only the custom domain for the gateway is shown and automatically used.

Yes, Indeed because once you add a custom domain then the default certificate is the certificate issued to the Gateway's custom domain and you will see Default SSL binding is ticked for that custom domain.

The value of Default SSL binding property can't be changed if you have one custom domain. In case of multiple custom domain you can designate the default certificate by selecting the checkbox of Default SSL binding property in portal. Please refer to the MS Doc which says below-

References-

How is the Gateway Url set in Azure APIM with multiple custom domains - Microsoft Q&A.