See my answer in your other thread for Hamburg - Copenhagen return

For Antwerp - Hamburg :

- IC Antwerpen-Centraal - Bruxelles-Nord 07:25 - 08:00 reservation not possible

- ICE Bruxelles-Nord - Köln Hbf 08:32 - 10:15

- ICE Köln Hbf - Hamburg Hbf 11:11 - 15:14

Book on bahn.com

- IC Hamburg Hbf - Fredericia 16:53 - 20:14

- IC Fredericia - Kobenhavn H 20:23 - 22:23

Book here : https://travel.b-europe.com/dsb-rail/en/reservation-only#Outbound

It'll be a long travel day anyway…

Or 2h20 earlier but it's an early departure (Antwerp 05:06 - Copenhagen 19:45).

I guess accommodation has already been booked everywhere ? A night in Hamburg would be best or you could've taken a night train from Hamburg

DB still can't book pass reservations online if they're mandatory, such as for the train Copenhagen-Hamburg. Sometimes it directly says "Reservation not possible” but here it just fails.

That means you need to split the booking. For Copenhagen-Hamburg, you can book on https://cd.cz/en, price is CZK 73 per person.

For Hamburg-München, it is still cheaper to book a reservation on bahn.com, €9 in total if you do it before midnight CET, afterwards it becomes €9,80 IIRC.

Thank you for all the guidance. I have made reservations for both routes.

Now the last part.. I have downloaded the PDF files for all reservations. So now how to add them to my Rail Planner App, link them to my Eurail Pass & activate them on the due date?

You do not add seat reservations anywhere. They are totally separate from the pass.

In order to get a ticket (QR-code) you add the journey to Rail Planner app. This can be done at any point. You need to this for every train you'd like to take.

Whenever asked on the train you show both the app (pass) and the PDF (seat reservation).

Great! So the Pass will activate itself automatically for that day when I get the QR code?

For example, I have a train booking for June 18th and I add the journey to the app today to get the QR Code. So now the pass will activate itself on June 18th?  Or do I need to manually do it?

There are 2 different things 

Adding a Journey to your Trip Means you are prepared for activation. (The slider at the right is grey) 

On the day of travel shortly before catching the train you click while you have a Internetconnection on the grey slider and turn it to the right. The slider becomes yellow which means the train is added to your pass and a travelday is activated. 

I recommend to activate a travelday as late as possible because you never know what can happen and worstcase you have a preactivated travelday that cant be canceled anymore