Ok, so this is an example of the multidropdown, with a box to add in “new” items, and the new item gets added to the selection (which is probably the tricky bit) when you add it.
https://blockexamples.bubbleapps.io/version-test/multitag?debug_mode=true
https://bubble.io/page?type=page&name=multitag&id=blockexamples&tab=tabs-2
The multidropdown works as standard, but it has a default set to a custom state. The custom state is kept in line with the values in the multidropdown, but gets added to when a new Artist is added, and then you reset the group to reset the values to add the new value.