Custom field not accessible through Odata query

Hi 

I have created a Custom field in one of our azure DevOps projects.

 Field Reference Name: Custom.TestingType. we are trying to access this field with an Odata query. but we get an error. Also custom field is not available in the metadata. 

https://analytics.dev.azure.com/companyName/myProjectName/_odata/v4.0-preview/$metadata