At Amsterdam Central Station, you can get a stamp when going though security/passport control on the Eurostar Amsterdam-London platform 15b.
After arriving on platform 15b, go to the Eurostar Departures building on the far end of the platform for a ticket check. After your ticket has been checked you can get a customs stamp at security/passport check. Ask an employee to point you in the right direction.
We advise you to be present at platform 15b around 60 minutes before departure. Ticket checks close 30 minutes before departure.