What do I need to do to get a reservation on a Thalys train from Antwerp Central to Amsterdam Central if I have a Benelux pass? Thank you.
Thalys do not accept the pass; you can get a Thalys passholder ticket which is likely to be more expensive than an advance purchase discounted publicly available ticket - if you can commit to an unchangeable ticket. Otherwise it is get out the wallet time. Passes aren't the deals they used to be.
If you can commit well in advance to a specific departure date and time, don't bother trying to buy a passholder fare ticket. A 2nd class passholder fare ticket costs €35 and Thalys limits the number of them it allocates for each train. The cheapest online advance purchase discount fare point-to-point ticket costs €29. Book up to 90 days in advance at thalys.com.
Similar to your last post, please spell out what you have planned on this trip. A pass is almost certainly not going to be worth the money, but we can't make solid recommendations unless we have a general idea of your itinerary.