There's a lot of handy info on the Seat61.com InterRail Pass page about this. Interestingly, much of it isn't positive about the InterRail discounted tickets...
A couple of key bits to pull out (but you'll really want to go and read the whole thing!)
Eurostar isn't covered .... The best and cheapest option is usually to forget your pass and buy a regular cheap Eurostar ticket well in advance at www.eurostar.com. These start from just £39 one way or £69 return .....
In theory, Eurostar offers a 'special' fare for InterRail passholders, £57 one way, £100 return, with changes to travel plans allowed, but as you can see, this is actually more expensive than the cheapest regular fare if you book a month or two in advance and commit to a specific date and train ......
but be warned, the number of passholder tickets is controlled by a quota and people have turned up at the station on a Sunday to be told there are no passholder places available on any Eurostar departure until Wednesday, and if they wanted to travel immediately they'd have to pay the full-price regular fare, a whopping £170 for immediate departure ......
Alternatively, take a ferry to mainland Europe: If you prefer to travel by ferry rather than Eurostar, or if you find all the cheap Eurostar fares sold out, there are several cheap ferry options.
Top tip: South Eastern Trains offer a £33.90 day trip ticket from London to Calais covering both train and ferry, which is cheaper even for a one-way trip than a £34 train ticket to Dover and a £30 ferry ticket on to Calais
So, assuming you don't just end up buying a normal cheap Eurostar ticket, and you don't decide on using the return half of a London-Calais day return by ferry
If you did want to book, then the InterRail website says that you can't book pass holder fares online. Instead, your options are to book by phone on +44 (0) 1233 617 575 (and I believe pay a fiver booking fee), or buy the ticket at a Eurostar terminal and hope that there are still some passholder fares left (otherwise it's going to get expensive!)