It's not possible to use global variables in User Macros, but this larger feature request would help to do that:
I would recommend that you vote on that and comment there with your usage case.
Let me know if you have any questions about that.
Badges are a great way to show off community activity, whether you’re a newbie or a Champion.Learn more
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG