From Heathrow you can take the direct Subway to London St Pancras (not included)
Or use the Elizabeth Line till London Farringdon and from there the ThamesLink service to St. Pancras :)
Just note from St.Pancras to Paris you need a reservation and have a similar procedure as on an airport Luggage check British and French Immigration Control :)
Reservation can be booked via Eurails Reservation Portal or via this website https://www.b-europe.com/EN/Booking/Pass#TravelWish
And to add: please provide ample of time to get from Heathrow to St. Pancras. Best calculate a bit of buffer in case your flight gets delayed. At least 4 hours are needed. So you just might want to spend the night in London to avoid that stress.
Getting from Heathrow to London St. Pancras takes at least an hour. Heathrow is big so, calculate an hour as well to get from the plane to collect your luggage. And add a delay buffer (Statistically planes are more often delayed (and much worse) than trains)
Half an hour before departure of the Eurostar you booked, you cannot enter any more.
Best is to arrive ca. 1h - 45 min in advance for your Eurostar departure to check your luggage and the double immigration check. Luckily the Eurostar is the only train in Europe with these strict procedures.