It seems it is not possible in QField (and probably not in QGIS either) for time being. Read more at Use the new relation widget when allowMultiple is enabled.
UPDATE QGIS 3.30: Multiselection and Completer is working: https://qgis.org/en/site/forusers/visualchangelog330/index.html#feature-multiple-selections-with-filtering-in-value-relation-widget