As soon as you board a train in the Netherlands it will count as an inbound/outbound travel day.

Which train would you like to take ?

- Eurostar to London ? Take the hourly IC-Direct as far as Brussels (more below) and board the Eurostar there

- Eurostar to Paris ? Buy a ticket as far as Antwerp or better -> take the hourly IC-Direct and check out with your OV-Chipkaart/contactless card in Breda. Board the Eurostar in Antwerp or Brussels

I think that's the right way to do it (as told by a Dutch friend) but @rvdborgt might be able to answer more precisely. Or should OP buy a ticket to the border point precisely ? (even though they shouldn't check tickets on that short leg to Noorderkempen)

Last thing : Eurostar trains have a passholder quota so book in advance.

Which journeys have you planned ? I'd say Eurostar is worth using an inbound/outbound day as you can board in Amsterdam/Rotterdam without issues.

Btw there's a trial going on and Dutch residents (along with other countries) get 3 inbound/outbound days. Activate your pass with a date far in the future if you wish to check.

If you don't want to use an inbound/outbound, then you need a ticket to the border. NS sell these as a ticket to a station behind the border, with 100% discount in Belgium or Germany. Can be bought online on nsinternational.com. (This doesn't work on Eurostar though.)

With such an international ticket, you don't check in or out anywhere.

If you have a subscription that is valid for free travel in the Netherlands, then you don't need any extra ticket and you still don't need to check in or out.

Ah yes they've got the Student OV which is valid until the border point.

That still exists indeed, but we can only guess what kind of ticket or subscription OP has. There are a lot of subscriptions valid until the border, e.g. the popular "Weekend Vrij”.