gFilterField = Left ( PortalRelationship::FieldToFilterFor ; Length ( gFilterField ) )
This will match Einstein if they type Ein
If you want it to match Einstein if they type 'stei' you can use PatternCount.
The details needed to get the portal to update keystroke by keystroke can be a bit tricky if that's what you want here.
See this demo file for search portals that update keystroke by keystroke. There's both a "starts with" and a "contains" example portal in the demo file.