I have a pipeline defined in the branches section triggered by certain branches. I would also like it to be manually triggerable, with a variable that configures execution in a certain way. In case of an automatic run from branches (where I can’t specify the variable manually) I would like the variable to be undefined or have a default value.
However, when I add a variables inside branches I am getting a 'Pipeline variables are only supported on custom pipelines' validation error.

Is there any other way to do that?
I’m migrating from GitLab pipelines where this was not a problem.