@Ravi_Ranjan We have yet to add a Companies resource to the REST API. For now, you’ll need to use the XML-RPC API.
NB: the /contacts resource does not allow manipulation of Companies.
The REST API will have separate resources for Contacts and Companies. They will likely end up looking/feeling different than what folks may be used to with the XML-RPC API.
So, in this case, hypothetically, AccountId would not be a field on Contacts but would rather be a field on Companies. (I use this as an illustration; it’s not guarantee of the future.)
But, good news: the REST API will soon have a resource for Companies, separate from Contacts. In the meantime, you can create Companies through the XML-RPC API .