How to POST Object Array with Retrofit?

Question

I send the data from the postman like this:

{"contacts":[
        {
        "idContact": "201",
        "name": "Joseph",
        "lastName": "Herrera",
        },
        {
        "idContact": "202",
        "name": "Lizzy",
        "lastName": "Martinez",
        }
    ]}

And it works correctly but I tried to POST from retrofit like this:

@FormUrlEncoded
@POST("saveContacts")
fun saveContact(
    @HeaderMap headers: Map<String, String>,
    @Field("contacts[]") contacts: List<Contact>
): Call<Response>

And this way:

@POST("saveContacts")
fun saveContact(
    @HeaderMap headers: Map<String, String>,
    @Body contacts: List<Contact>
): Call<Response>

But both ways doesn't work for me. i get a bath response from API:

local.ERROR: Illegal string offset 'idContact' {"userId":1,"exception":"[object] (ErrorException(code: 0): Illegal string offset 'idContact' at

or

local.ERROR: Argument 1 passed to App\Services\ListingService::create() must be of the type array, null given

respectively.

Contacts is a class with the field specified. I send the Info to retrofit Function like this:

    val contacts= listOf(contact1,contact2)
    RetrofitClient.instance.saveContact(
        headers = headerMap,
        contacts = contacts
    )

What is the correct way to post this kind of data?