I was trying add this to my Lead data before to create:
"contacts" => [
[
"first_name" => "Contact 6",
"custom_fields_values" => [
[
"field_id" => 2960404,
"values" => [
[
"value" => "3333333"
]
]
]
],
]
]
But, once the lead was created, the lead has not a contact, just empty.