Creates and adds the contacts to the sequence in the specified step.
A new contact will only be created if there is no contact for the organization corresponding to the parameters in the to list.
If a new contact is created, it also creates a new conversation with the from number channel. If there is an existing contact but no conversation, a new conversation will be created.
If there is more than one contact in the to list, the contacts will be tagged with an upload tag with a timestamp corresponding to the time of the request.
If contact is already in the specified sequence step it will be reprocessed and if it is active or pending in another step of the sequence it will be ignored.
Besides that, if contact is already in the sequence its up to the sequence settings if the contact will be added again or ignored.
When custom_data is provided, the contacts will be associated with a custom object. The custom object and its custom properties must be created before the contacts are added to the sequence. When specifying custom data, the resource field must be set to contact or step_contact in order to associate the custom data with the contact or the step contact respectively.
The process will run in the background, therefore a 202 Accepted status is initially returned on success.