Hoek Van Holland to Harlaam transportation (dutch flyer)

I don't know what it will say on your ticket so don't take this as gospel.

I do know the Dutch train routes between the train stations in western Netherlands, the area known as the Randstad. The main line goes directly between Rotterdam and Amsterdam Centraal via den Haag, Leiden, Schiphol Airport and Amsterdam Centraal. Note that your ticket will not be valid on the dark red coloured Thalys train but it is likely to be valid on the new InterCity Direct which only calls at Schiphol Airport on the way from Rotterdam Centraal to Amsterdam Centraal and does it in 35 minutes versus about an hour on normal trains.

If your ticket shows that you have to travel via route Schiphol then you will have travel through to Amsterdam Centraal and then change to one of the very frequent 15 minute trains as far as Haarlem.

On the other hand, if your ticket allows you to travel any reasonable route to Amsterdam, you can take a slower train which stops at Leiden and change there to a train on the branch line loop (to Amsterdam Centraal) via or to Haarlem. It will probably make 2 or 3 other stops on the way to Haarlem. It is fine to leave the train at Haarlem as that is a valid route to Amsterdam as long as it not restricted by the ticket.

Although changing at Leiden and avoiding Amsterdam is a more direct route it is likely to take somewhat longer than a fast train to Amsterdam Centraal and an easy change to Haarlem.

I don't have any idea how the replacement bus is working out nor how long or reliable it is. When it turns back to a train, albeit a Metro not a heavy rail, later in the year it will likely be more predictable.

None of the trains mentioned above except the Thalys, which you can't use anyway, have reservations. Look at the departure boards and take the next train heading your way, checking that it isn't a stoptrain (all stops) unless it is the Haarlem leg.

If you have a smartphone and available data having either the 9292 or NS Reisplanner apps you can watch in real time, get platform numbers and check connections. The Reisplanner app even shows you a photo of what the train looks like. Better views from upstairs on a double decker - free hint.

Don't worry about the train connection. From Hoek van Holland you'll be taken to a bus, which replaces the normal train connection. This will bring you to the railway station of Schiedam. There you can pick up a train to Haarlem. These trains run every 15 mins, alternatingly direct or with a change at Leiden. Consult the train planner of the Dutch railways for the train(s) you need.
Train services in the Netherlands don't go with reservations.
Mind the spelling and pronunciation: it is Haarlem, more or less pronounced like the Harlem in NYC, which was called after it.

Here is a link to information on the replacement bus service -- https://community.ricksteves.com/travel-forum/transportation/hoek-van-holland-rotterdam-replacement-buses