Lead Process auto-populate "Existing Account"

Hi,

When creating a lead upon entering the Existing Contact information I want to auto-populate the Existing Account field in the BPF. But I try with Business Rule and Workflow, but the Existing Account and Contact fields are not listed anywhere to use in the Business Rule or Workflow.. 

So how do I get a onchange event on the Existing Contact field so I can generate som js to auto-populate the account?

thanks for any help

regards

Martin