How to add a value to a new, blank custom field in customSchema

I'm trying to add a value to a custom field within a customSchema using Google Apps Script. Whether I add the customSchema using the Google Admin Console, or through Apps Script, I run into the same problem: The customSchema can't be found with the Admin SDK with Apps Script until I put a value in manually using GAC.

function updateCustomSchemaData(userEmail, schemaName, fieldName, newValue) {
  // userEmail = 'user@email.com'
  // schemaName = 'User_Information'
  // fieldName = 'Site_Code'
  // newValue = 'test'
  try {
    var user = AdminDirectory.Users.get(userEmail);
    var schemaData = user.customSchemas[schemaName];
    schemaData[fieldName] = newValue;
    AdminDirectory.Users.update(user, userEmail);
  } catch (error) {
    Logger.log("Error updating custom schema data: " + error);
  }
}

I confirm through GAC that the field exists before running updateCustomSchemaData.

When running the code, I get the following. Error updating custom schema data: TypeError: Cannot read properties of undefined (reading 'User_Information')

If I check the output of user, it doesn't contain any customSchema.

If I edit the field in GAC and add a value to that customSchema manually, var user = AdminDirectory.Users.get(userEmail); will have the customSchema in it.

  "customSchemas": {
    "User_Information": {
      "Site_Code": "This is a test"
    }
  },

I can confirm that the schema does exist, since I have a separate, working function to create the schema if it doesn't (using AdminDirectory.schemas.insert), and as long as the schema is in GAC, even if it's not in user, I get this:

Failed addSchema with error API call to directory.schemas.insert failed with error: Entity Already Exists: resource.schemaName

TL;DR At first glance, a newly created custom field in a customSchema can't be seen by AdminDirectory.Users.get, and AdminDirectory.Users.update can't a value to the field. I'm hoping this is incorrect, and I just don't know how.

Please help me find a way to update a value in a newly created, blank custom field in a customSchema.

UPDATE #1: Current code, doesn't error, but doesn't add the value to Site Code.

function updateUser() {
  // update individual user, given email, to make Site Code match buildingId
  const user1Email = "user1@email.org";
  //const user2Email = 'user2@email.org';
  try {
    var user = getUser(user1Email);
    var userId = user.customerId
    const myFunction = () => {
        AdminDirectory.Users.update({
          "customSchemas": {
            "User_Information": {
              "Site_Code": "This is a test"
            }
          }
        }, user1Email);
      myFunction();
    }
    var user = getUser(user1Email);
  } catch (err) {
    console.log('Failed with error %s', err.message);
  }
}

The goal is to add a value to a custom field within a customSchema using Google Apps Script created by Method: schemas.insert. The Error updating custom schema data: TypeError: Cannot read properties of undefined (reading 'User_Information') is because Method: users.get doesn't return customSchemas if there's no value to it even if projection is set to full.

To resolve this, use Method: users.update directly like this:

const myFunction = () => {
  AdminDirectory.Users.update({
    "customSchemas": {
      "User_Information": {
        "Site_Code": "This is a test"
      }
    }
  }, "user@email.com");
}

Note: I noticed that Method: users.get only returns a value for customSchemas if projection is set to full and there's a value to it.

OUTPUT

BEFORE

AFTER