How to determine formatter expression for custom assets field?

My app is unable to update the custom field for Jira asset objects and I am receiving a “This API works only with fields provided by Forge apps.” error. I believe it is due to my manifest.yml file being set up incorrectly, specifically the formatter expression value:

    - key: custom-field-10063
      name: Passenger
      description: Custom field to store passenger information
      type: object
        expression: "`${objectType.objectAttribute}`"

The documentation for how to make the Jira expression is lacking, how would I go about generating this Jira expression?

I gave up on however this method is supposed to work and attached my asset object to a custom field via a PUT issue call. This seems to work even without the jira:customField: in my manifest.yml.

const endpointUrl = `${issueId}`;
  const bodyData = JSON.stringify({
    "fields": {
      "customfield_number": [
          "workspaceId": workspaceId,
          "id": `${workspaceId}:${objectID}`,
          "objectId": objectID
  try {
    const response = await fetch(endpointUrl, {
      method: 'PUT',
      headers: {
        'Authorization': authHeader,
        'Accept': 'application/json',
        'Content-Type': 'application/json'
      body: bodyData