Currently, there are limitations with multi-select lookup, you can not use it in subform for example. As a workaround we have to use a multi-select field. However, any new options have to be manually added to the field which is not optimum.

It would be nice to have an Api / Deluge Task, that allows adding new options via functions to that specific field.

it would require to know the field ID and send the options as a list