How to add a default pipeline variable for a task group?

I have a task group that's used in 20+ release pipelines, and one of the tasks is a PowerShell "backup environment" task. I'm using classic pipelines.

Is there a way to add a default pipeline variable "RunBackup = true" or modify the task group so that when you create a new release, every pipeline will have a "RunBackup" parameter that is settable?

I added "$(RunBackup)" as an argument to the PS task, and it shows up as a parameter, but it requires me to modify every single release pipeline to manually add a release variable.