Currently, there is no way to append/add values to a checkbox list type of custom field as both the "add" and "update" functions throw an error within the advanced JSON editor when editing a create/edit issue action that the custom field type is incompatible.

Allowing these functions to work with a checkbox list type custom field would reduce the need for users to use some sort of a work around to do so (especially when considering the "COPY" functionality works when attempting to append a value to that custom field, using the advanced JSON editor does not when trying to set a value that isn't found in another field).