You must add a new Forge Runtime. It's on the JBDS docs: https://access.redhat.com/knowledge/docs/en-US/JBoss_Developer_Studio/5.0/html/Forge_Reference_Guide/chap-Forge_Reference_Guide-Preferences.html
I have the same issue. I upgraded JBoss Studio to Version: 10.4.0.GA. However when I launch forge from JBoss Studio (Ctrl +4) in the lower left hand it says Jboss Forge 3.6.1 Final. I want to upgrade to JBoss Forge 3.8.1. I went to Eclipse Market place but could not find Forge. I see that you have a link above, however that link no longer works.
Any help is greatly appreciated.
That was valid for Forge 1.x. For Forge 3.x onwards the runtime is now embedded in the plugin. Forge 3.8.1.Final was not updated in the plugin runtime and should be done this January.