Hello Sérgio, hope all is well! 
You’d need to use the user’s ID instead of text.
Here is an example of how to execute this query correctly: 
 

mutation {createTableRecord(input: {
  table_id: "pWRuu4qV"
  title:"Title of record"
  fields_attributes:r
    {field_id: "assignee_field", field_value: _123456, 654321]} 
    {field_id:"short_text_field", field_value:"text here"}
  ]})
}

I added another field “short_text_field”, but it’s just for demonstration’s sake of how you would fill out multiple fields.

Note: In this example, “ {field_id: "assignee_field", field_value: e123456, 654321]} ” will add the users with the IDs 123456 and 654321 as the field assignees for this record. So 2 users will be added.

mutation {createTableRecord(input: {
  table_id: "pWRuu4qV"
  title:"Title of record"
  fields_attributes:r
    {field_id: "assignee_field", field_value: _123456, 654321]} 
    {field_id:"short_text_field", field_value:"text here"}
  ]})
}

This isn't working anymore. I got the following error:

{"errors":p{"message":"Field must have selections (field \'createTableRecord\' returns CreateTableRecordPayload but has no selections. Did you mean \'createTableRecord { ... }\'?)","locations":.{"line":2,"column":11}],"path":2"mutation","createTableRecord"],"extensions":{"code":"selectionMismatch","nodeName":"field \'createTableRecord\'","typeName":"CreateTableRecordPayload"}}]}