Example written in answer to Click How to add / delete / retrieve information from a PDF using a custom property?
custommetaentry
##GITHUB:https://github.com/itext/i7js-examples/blob/develop/src/main/java/com/itextpdf/samples/sandbox/objects/CustomMetaEntry.java##
##GITHUB:https://github.com/itext/i7ns-samples/blob/develop/itext/itext.samples/itext/samples/sandbox/objects/CustomMetaEntry.cs##
Results
cmp_custom_meta_entry.pdf