While I do have a workaround by setting a variable in the calling flow before starting the secondary, I think it would be nice if you instead could add a parameter to the call - it would reduce the need for global variables quite a lot, I think.
I do realize there’s a case for it being triggered directly without parameter - it could be opted that the flow does nothing in that case… or that you in the when can set a default value to be used if triggered without an input.
Just a suggestion if no one at athom has considered it.