Maybe you can already book in Bulgaria, but certainly in Istanbul. Book on arrival, either at Halkali or Sirkeci station (Sirkeci is in the city centre). There are no seats, only sleepers and couchettes.

Note that this is a direct train Halkali-Sofia. No change is necessary. If you have a flexipass, then enter it manually, to prevent that you use 2 travel days.

What is a Flexipass?
How can I enter the journey manually?

When you have X days within 1 or 2 months that you can use when you need them. The other type of pass is the continuous pass.

https://www.seat61.com/how-to-use-an-interrail-pass.htm

https://www.interrail.eu/en/interrail-passes/what-is-interrail/how-use-your-interrail-pass

In the results of the planner, scroll to the bottom and tap on "Add it manually” / "Füge sie manuell hinzu”.