Pitch consultancy AAR is helping the developer in its hunt for a media agency.
The process is currently in the early stages, with pitches due in the New Year.
At 150,000 square metres, Westfield London will be the biggest shopping centre in Greater London, rivalling traditional retail districts such as Oxford Street and Covent Garden.
The centre will include four main stores, more than 265 speciality shops, restaurants, a cinema, health and beauty retreat and gym.
According to Westfield, the new centre has a catchment area of 2.7 million people, with an estimated retail spend of £5bn.
The development is part of a major drive to regenerate the area in West London.
Westfield London is part of the Westfield property group, the world's largest retail property group, with a portfolio of 119 shopping centres in Australia, the US, New Zealand and the UK.