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It may need a custom field type added to the dynamic objects menu that would allow the user to choose an existing but also enter a custom ID for new blocks. Something like these:
I was planning on adding a helper for something like this, and short-listed a couple libraries on Github, but they don't seem to allow custom text input except in a "tagging" mode, which isn't quite what we would want.
Would one of those be the best way to offer a select list that lets users suggest new list items?