Update custom User field via GraphQL API

I can't find sufficient documentation or examples to determine the right way to do this.

I have custom_field_metas with id: "26850" and can query a user's value for this field.

How can I do something like this? or what's the right way to update custom fields on the user object?

mutation {
  update_multiple_users(user_updates: [
    {
      user_id: 12345,
      user_attribute_updates:{
        custom: {
            custom_field_meta_id: 64895,
            value: "TEST2"
          }
      }
    }
  ])
}