At the moment it is not possible to show Forge custom fields in the JSM portal. I found a similar feature request ticket for custom field types if you would like to follow it: [FRGE-778] - Ecosystem Jira
I have also asked the relevant team to see if any work is planned to support Forge custom fields in the JSM portal. I will update here and on the FRGE ticket with any new information as I get it 
Hope that helps!