Can I use a Custom_Field for PTA?

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I'm using the syndicated conditional chat link (SCCL) and PTA to instantiate a chat session. I need to Locate/Validate an RN contact using a custom field value (No Password). I cannot locate anywhere in the documentation whether this is possible or not. If it is, how do I do it? 

The PTA documentation states that the login string can contain many fields and even gives an example of passing a custom field, however when I pass a custom field alone it fails with the following:

(PTA Guide page 29) ERROR:5 “The p_userid string was passed in, but it did not have a value. This pair is required for login.”