Setting new Drive Labels With Selections to a specific file using Google Apps Script

The question & answer here was very helpful:
https://stackoverflow.com/q/76256164/22038334

I'm looking to take this to the next level, and set a label on a known file that also has Selection. So in my case, I have these label selections and their IDs:

3ZB971298F: Archived

9745AD2361: Unclassified

I have

  var AddLabel = Drive.newModifyLabelsRequest()
    .setLabelModifications(
      Drive.newLabelModification()
        .setLabelId(labelId)
    );

And this works well to set the label that's defined by lableId. But it's not complete, because it's missing the Selection. How can I add a selection, let's assume "Archived" with the ID as above.

I've read through the documentation (https://developers.google.com/drive/api/reference/rest/v2/files/modifyLabels#FieldModification), but not quite getting it. Thanks.

I couldn't find documentation on setting Selection. I tried the typeahead to see if something useful could come up, but no luck. There is surprisingly very little documentation specific to GAS.

I've figured out the solution after some trial and error. These methods and parameters are not well documented for Apps Scripts. Please note, to acquire the field id and the field selection id, I had to fetch the label of a sample file that has the field and selection value of interest (Drive.Files.listLabels(fileId)) and then manually inspect the returned object to find the fieldId and fieldValue selectionId.

Here is the solution that works for me:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

var fileId = ""
var labelId = ""
var fieldId = ""
var fieldValue = ""

Logger.log(DriveApp.getFileById(fileId).getName());

// Add Label To The File (Fields will be empty)
var AddLabel =Drive.newModifyLabelsRequest()
AddLabel.setLabelModifications(Drive.newLabelModification().setLabelId(labelId));

// Commit
var output = Drive.Files.modifyLabels(AddLabel,fileId);
Logger.log("%s",output);

// Set field of file
var fieldModification = Drive.newLabelFieldModification();
fieldModification.fieldId = fieldId;
fieldModification.setSetSelectionValues([fieldValue]);

var modifyLabelsRequest = Drive.newModifyLabelsRequest();
modifyLabelsRequest.labelModifications = Drive.newLabelModification()
modifyLabelsRequest.labelModifications.labelId = labelId;
modifyLabelsRequest.labelModifications.setFieldModifications([fieldModification]);

// Commit
var output = Drive.Files.modifyLabels(modifyLabelsRequest,fileId);
Logger.log(output);

<!-- end snippet -->

Wyler's answer worked great. I ended up finding this solution at about the same time, so posting it here as well:

 var labelId = "";
 var fieldId = "";
 var selectionId = "";

 const request = {
    "kind": "drive#modifyLabelsRequest",
    "labelModifications": [
      {
        "kind": "drive#labelModification",
        "labelId": labelId,
        "fieldModifications": [
          {
            "kind": "drive#labelFieldModification",
            "fieldId": fieldId,
            "setSelectionValues": [
              selectionId
            ],
            "unsetValues": false
          }
        ],
        "removeLabel": false
      }
    ]
  }
  var output = Drive.Files.modifyLabels(request, fileId);
  Logger.log('Label Modified output: ', output.modifiedLabels);
  Logger.log('File: ' + fileId + ' Label: ' + selectionName + '/' + selectionId);