Forums will be making significant UX changes to address key usability improvements surrounding search, discoverability and navigation.
To learn more about these changes please visit the announcement which can be found
I think you should use the N9 (Reference Information) Segment.
The first element (N901) is the Qualifier, where you need to agree with the receiver of the document, what qualifier should be used for credit card information. In this example, E4 is used for "Credit Card Number", so you would have the following: