MID(Field, FIND("[", Field) + 1, FIND("]", Field) - FIND("[", Field) - 1)
I think this should work for your use case. You'll just need to replace "Field" with whatever the name of your Title Field is.
As for that trigger a drop down menu, I'm not sure I completely understand, but I'll provide an answer based on what I assume you're asking. If you're wanting that to create/choose an option in a single select field, you'd need an automation. You'd want it to trigger when the formula field is updated, then you'd want it to update the original record's drop down field to the text in the formula field.
Ahh makes all the sense, thank yu!
@totoporo Please make sure to mark the solution so people know that it's been solved. Not sure if you did or not as it's showing me a clickable button to see the solution in thread even though nothing is marked as "Solved" on my end. Might be a bug, but wanted to remind you to mark it as solution if you didn't already 🙂