I have noticed that for Service request, the placeholder for ticket.id returns me something like “SR-5” rather than the ID of the ticket which is for example “5”.

….reshservice.com/api/v2/tickets/{{ticket.id}}/requested_items

So it seems it gets converted to: 

....reshservice.com/api/v2/tickets/SR-5/requested_items

Any idea?

You need to use the placeholder for ticket ID. if you check in the list of placeholders you can see it a bit down the list. 

@Daniel Söderlund “{ticket.id_numeric}}” worked :)