So glad you were able to figure out a workaround here!
At the moment, the subitems are still in beta, and with this in mind it does mean that not all platform features are currently compatible with the subitems.
Currently, does include the exact formula you’re looking to create I’m afraid.
As @basdebruin and @rob have suggested, General Caster would be a good workaround here, although it sounds like the Webhooks worked out for you better here!
I can definitely appreciate how this would be a useful addition to the platform for yourself, and I’m sure for other teams too, so I would highly recommend opening a thread in our Feature Request section.
Here other users who would also like this added to the platform will be able to upvote the feature!
You can find this here: Feature feedback - monday Community