Using an app variable in webhook

Hey all!

As a programmer, I love it when I can define a constant that I can use everywhere. That way I only change one thing when/if that thing changes.
App variables are these beautiful constants.

However, I can not find a way to use app variables in a webhook, which feels like an oversight to me.
Currently, I am trying to use an app variable in the URL of the webhook. 
First off, it's a little annoying because the webhook URL must start with https://, second, it's annoying because I can't use app variables here. As far as I know.

I tried creating a field that was just the app constant, because I do have access to fields and their values in a webhook, but it altered the slashes and turned them into % things that the URL didn't know how to handle. 

So, how can I use an app variable in a webhook?
Or is there some other way to have a constant? It would be even better actually if I could share it between apps. =)

Thanks for the help!

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- Sterling Long!
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