It states that you cannot share web tools to AGOL unfortunately, but you can add the URL published in Enterprise as a geoprocessing service item in AGOL.
Quote here:
You cannot share a web tool to ArcGIS Online, but you can add the web tool URL published in ArcGIS Enterprise as a geoprocessing service item in ArcGIS Online. See Add items from web in the ArcGIS Online help for more information. You can only consume the added item from ArcGIS Pro, not from ArcGIS Online.
