Is it possible to add polygon or line feature layers in the GeoForm application in ArcGIS Online?

Answer

No, it is not possible to add polygon or line feature layers in the GeoForm application in ArcGIS Online. This is a known limitation as polygon and line feature layers are not supported by the GeoForm application. Only editable point feature layers that are hosted feature services or an ArcGIS Server feature service can be added.

Attempting to add an editable and hosted polygon or line feature layer in the GeoForm application returns the following warning message:

Warning:   
The selected webmap does not contain a valid layer to use. Please add an editable FeatureLayer into your webmap to continue. For More Information please refer to What is Feature Service?

Although the functionality to support polygon and line feature layers in the GeoForm application template is not included in the current release of ArcGIS Online, there are plans for it to be implemented in a future release. Visit the ArcGIS Ideas page, GeoNet: Polygon Function in GeoForm to support the enhancement in future releases. Refer to ArcGIS Online: What's new for updates.