The current MMVP approach in Citoid implemented in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Citoid/+/1037473 still needs to provide an option to set the actual content of the extension. This should happen by showing a dialog after choosing the parent reference in the onExtendsSearchResultsChoose handler.
We decided on not re-using the base dialog from Cite at this point because it's to tangled with the rest of the logic in the ReferenceDialog class. For now just try to create a simple window with an VE target textarea.