No, Forge provides no UI for assigning anything to these fields before sending the issue off to the server.
However, some of the supported forges support commands that you can insert into the body of a new issue or merge request. Gitlab calls them “quick actions”; you’ll have to look through your forge’s help to see what they support. On Gitlab, putting “/label ~foo” in the body of a new issue will cause the Gitlab server to add the label called “foo” to the issue.
If that’s not available on your forge, you’ll need to view the issue or merge request after creating it, then put the point on the field you would like to edit (“Milestone”, “Labels”, “Marks”, a comment, etc), and type
C-c C-e to edit it. This is all documented in chapter 4.3.4 Editing Topics and Posts of the Forge manual. The manual is also available inside Emacs using the Info viewer (
C-h i). Doing this should not require refetching the issue; it certainly doesn’t when I use Forge to create issues on Gitlab.