workflow_dispatch only from default branch

I like to set Github Actions to only run when manually triggered, that is, using workflow_dispatch

As that page says,  "To trigger the workflow_dispatch event, your workflow must be in the default branch."

But we can change the default branch - like we may want to do for forks etc - Repository --> Settings --> Default branch (choose by clicking the "choose another branch" button which has two opposing arrows as an icon)