I have a eurail Global Pass, now how do I make a reservation on a eurostar train

Isn't support part of the reason you spent such a massive amount of money on passes? Call Eurail and get some benefit out of the pass.

https://www.eurail.com/en/europe-by-train/high-speed-trains/eurostar

That is another big incentive to getting a Global Pass now as it is valid on the EuroStar plus valid in Poland. This didn't used to be the case.

Emily's response is absolutely correct.

The easiest way is to call whoever sold you the pass and have them make the reservations for you. This will involve additional fees.

If your plans are set, you might as well go ahead and make all the reservations you need while you're at it.

If you are getting to Brussels by train from France you will need to buy reservations on any TGV or Thalys trains. TGV reservations for pass holders are rationed. Thalys reservations aren't rationed for pass holders but are very expensive.

EDIT - oops, corrected

Nice typo Nigel.

You can also buy the reservations here: https://ricksteves.raileurope.com/us/rail/point_to_point/triprequest.htm. Don't call unless you really can't figure out the web site(s), since phone service costs more.