Masking HTTP header data is a global setting, configured in the RUM Console by opening the AMD configuration. Navigate to Global->Front-End-Monitoring->Web->HTTP->Sequenced Transactions and Header Data. Depending on the version of DCRUM you may be in the correct place or have to also open the General settings in this section.
In the Parameter Masks add a mask for the entry for the text you wish to match. Add a * for what you want masked. For example, if you see (or know) that
is in the header - you mask it by adding an entry:
In your example, I think either "header_name*" or "header_name:*" would work. The amd will match the text and then "*" the rest of the string up to a header parameter separator character.