You have to buy a normal Ticket for this train. Online via https://www.malpensaexpress.it/en/ or using ticket machines in the train station. Also possible is to pay with pay and go, with your credit card. 

There is no need to pre-book this, it is a fixed rate of €13 no matter when you buy the ticket.

Also that connection Milano P.G - Milano Centrale you have from the planner is way too short.

In fact the train from the airport continues to Milano Centrale arriving at 09.07 which is way too tight to reliably expect to catch a 09.10 departure.

As is stated here often, do not use the interrail app to plan journeys, use planners from local train operators or those from reputable operators that cover most of Europe such as bahn.com and oebb.at

There are also direct Malpensa Express trains from Milan Malpensa Airport to Milan Centrale station.

The one in that plan is a direct Airport - Centrale train. The planner should not be suggesting leaving it at Milano P.G. and using an unlisted alternative to get to Centrale, it certainly shouldn’t be suggesting doing it in 15 mns.

There is obviously something wrong with the way it is calculating connections between these stations if it is rejecting a direct connection between 2 trains at Centrale and suggesting the inter-station transfer instead.

I don’t know how Italian station to station transfers are usually calculated, certainly in UK it is minimum allowed connection time at each station added together and a conservative estimate of station - station journey by the stated mode which leads to times long enough for the slowest traveler to comfortably make a connection.

In this case-as a kind of heritage from the past, there is no set time in between, as it was/were 2 different companies. You could not even buy through tickets, so all obsolete. MP-EXp has only 1st cl for ticketing. There are also frequent buses direct for 10€.