There is no option to add a new feature for a feature service layer participating as the destination in a simple relationship class within ArcGIS Field Maps.
Childs in a Feature to Feature relationship are hidden by default within the app to mitigate the potential collection of orphaned records. Use the Show Related Types setting in Field Maps or Collector to allow the creation of all feature types from the Collect action. Show Related Types is "Off" by default.
https://doc.arcgis.com/en/field-maps/android/help/configure-field-maps.htm#ESRI_SECTION1_9422B8595D58472F88132BF0ADA89FF0