Querying address book entries by User Defined Fields comes with some peculiarities that we would like to cover up in this article: 

- In order to query for a User Defined Field that does not contain any special characters, spaces etc. in its structure, the format is the following: 

Udf.VWRmRGVmaW5pdGlvbgk2MDAzMAkw='5'

- User Defined Fields that contain special characters need to be rounded with backticks in a query to preserve their structure as it is when passing the request to the provider. For example: 

`Udf.$NAME(<<field_name>>)`='60003'