Zoho CRM API fetch custom fields

We are trying to access the custom fields of CONTACT module and need to updated the field data based on that. We have a multi-line custom field.

We are trying to fetch the Contacts using the getRecordById and searchRecords API. We are getting the contact details in the response but it is not showing any custom field data in the response.

Please provide us information on how to get the custom field data and update it using the API.