After referring the documentation, I don’t see an issue in your approach and ideally it should work, please allow us some time to check with the concerned team and get back to you!
I worked with Freshdesk team to learn what could be the issue. As they pointed me to above piece of documentation, there seems to be a relevant field called ‘custom_number’ to perform query on custom fields instead of label in general.
Can you please try doing a query in the following way and let us know if this is working?
I am tried with custom_number instead of custom_string, it’s working fine.
We want to mention that field name to filter tickets?. Because we pass multiple parameters to get at single query it efficient, we can avoid the time consumption of pagination request.
Looks like same API response might not have label information that you need. It may cost you more API calls for now. I am still waiting for confirmation from Freshdesk team.