If you want to book on the Interrail website, you'll need to create a trip there and add yourself (and any others) to that trip with your pass details. There's no link between a trip you create there and the trip in your app. Reservations are entirely separate from the pass. The only link is your usually name.

But it's best to avoid the Interrail website for reservations if you can. It's not so reliable and more expensive.

Here's an overview of where's best to book:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Book Eurostar on www.raileurope.com, so you don't have extra booking fees.

Thank you @rvdborgt for your help. I was getting confused between the various different websites, apps and booking systems. It doesn’t seem v easy to book reservations directly on the rail companies websites, although I realise some of the apps let you do this. I had also seen some not so positive reviews about people’s experiences on Rail Europe, but I guess that will happen on any booking system. I only needed three reservations so in the end I used the Interrail website after checking the actual times elsewhere and all seem to have gone through ok. Many thanks