This partnership has long been limited to a few NEC destinations around Newark (see
the earlier thread/wiki), and does not apply to other flights. Yes, you can type your AGR number into a UA reservation, but I assure you that nothing will post unless it involves one of those few destinations.
The UA chart title clearly states "Mileage accrual chart for Amtrak-operated segments". The only "Amtrak-operated segments" that UA sells are those listed under the first bullet point: EWR to New Haven, Stamford, Philadelphia 30th St, Wilmington. For an example, see
this flight search for ZFV-LAX (ZFV=Phila 30th).
The AGR site clearly states "POINTS ON
QUALIFYING UNITED FLIGHTS" (emphasis added) and continues "take Amtrak to connect to or from Newark Liberty..."