type User @model {
  id: ID! @isUnique
  email: String
  firstName: String
  lastName: String
  posts: [Post!]! @relation(name: "PostOwner")
  createdAt: DateTime!
  updatedAt: DateTime!
}

type Post @model {
  id: ID! @isUnique
  title: String!
  contents: String!
  owner: [User!]! @relation(name: "PostOwner")
  createdAt: DateTime!
  updatedAt: DateTime!
}

Do I have to manually set the owner when creating a post?

It would be

mutation { ... userId: "iweiwdgw4u73g3hg" }

ahhh I see, thank you