Specifically, I want to use the "Add to subtree" feature.
Thanks for BeOrg!
This isn't currently supported. The Share extension has limited access to your beorg files and doesn't actually save directly to a file but creates a temporary file which then gets merged the next time beorg starts (potentially in the background when it refreshes data).
You are best looking at creating a Shortcut that uses the beorg URL scheme. There are a number of learning articles on the beorg website showing examples of these https://beorg.app/learning/.
Also need this feature. Maybe you can add a setting about which file and subtree to capture, and merge the temporary file there when beorg start. This can also affect when I press the '+' button at the agenda.