@JakeSkinner thank you for looking into it. Yes, the code is:
---------------
poly_layer = gis.content.get("e7e052cc4ad044798c4f5a6c893f20a3")
target_FL = poly_layer.layers[0]
status= target_FL.append(item_id="0472cbe2496c48e580f56d5c04155182", upload_format='shapefile',
field_mappings=[{"name":"OBJECTID", "source":"FID"}],
append_fields=["OBJECTID"])
status
-------------------
Here, target_FL is feature layer on ArcGIS Portal & "0472cbe2496c48e580f56d5c04155182" is itemID of a shapefile.
Also, if we use featureCollection instead of shapefile, it doesn't work either.
Reference: https://developers.arcgis.com/python/guide/appending-features/