Hide elements on the portal based on URL parameter check , how to use a config to control

Content

We aim to use consistent pages as much as possible between our interfaces as we have a large number of them so trying to substitute part of the url parameter_check condition with a config to control the answer ID per interface.

This is working great:

<rn:condition url_parameter_check="a_id == '1642'">
            <rn:condition_else/>
<rn:widget path="custom/Feedback/CustomAnswerFeedback" label_title="#rn:msg:IS_THIS_ANSWER_HELPFUL_LBL#" dialog_threshold="#rn:config:CUSTOM_CFG_ANSWER_FEEDBACK_DIALOG_THRESHOLD_ON_CUSTOMER_PORTAL#" />
        </rn:condition>

But trying to put a config option in place of the 1642 is not working and giving a syntax error, I have tried a text and Integer config option but no luck.