Meta data is a hierarchy, so you cannot write everything using only paths, you'll have to use intermediate BitmapMetadata objects.

The official documentation for all this is located here: Native Image Format Metadata Queries which is part of WIC or Windows Imaging Component documentation, the underlying Windows imaging technology that WPF uses.

The doc says this for TIFFs:

/ifd/iptc or /ifd/{ushort=33723} / IPTC / VT_UNKNOWN - A query reader/writer

The obscure VT_UNKNOWN (for "Variant Type IUnknown") in fact means iptc is an object that can read and write meta data (aka: a BitmapMetadata in WPF parlance), the start of a meta data sub tree.

So what you must do is something like this:

Dim iptc As BitmapMetadata = New BitmapMetadata("iptc")
iptc.SetQuery("/{str=Headline}", "TEST_HEADLINE")
oMetaData.SetQuery("/ifd/iptc", iptc)