Yes!!! Now it works!

I will leave here the mutation that worked out.
Mutation:
mutation ($author: [AddAuthorInput!]!) {
addAuthor(input: $author) {
author{
id name
}
}
}
Variables:
{
"author": [
{
"id": "id1",
"name": "George Lucas"
},
{
"id": "id2",
"name": "George Lucas2"
}
]
}
Thank you for your help @rajas !