I see plenty availability for Friday 23/06, and Monday 26/06. I doubt it would be fully booked out so far in advance. 

Could be that the schedule is unsure because of planned engineering works on the line and therefore not open for booking.

Good alternative is to book until Brussels, and take reservation free hourly IC trains towards the Netherlands, but probably calculate some extra travel time too, if there's works on the line. 

Thank you this is my first time taking eurostar and I’m not sure how long to wait to make that change fearing it might get sold out. Jimb

As soon as you know you wa't to take the Eurostar, try to book it. Pass holder seats can sell out. It's the only train in Europe that has no proper alternative too. In this case, I would book to Brussels (if possible) and continue your trip with the IC towards the Netherlands. 

Sounds good what area of Brussels does the eurostar go to for my connection? Thank you

Eurostar and the IC to Amsterdam serve Brussels Midi.

If you look on the Eurostar.com website you can see there is no availability on that weekend to Amsterdam. It may be that the timetable is not finalized yet or there may be works meaning Eurostar will not run past Brussels on those days.