title: Is it possible to create a custom namespace tag in XMP Dublin core metadata?
body: <p>I am curious to know whether it is possible to create a custom namespace for the <a href="https://en.wikipedia.org/wiki/Dublin_Core">XMP Dublin Core metadata</a>?</p>
  
  <p>For example If I want to add a tag like <code>mytest</code></p>
  
  <p>I can write the XMP Metadata standard tags such as <code>description</code> however I can't write my own tag</p>
  
  <pre><code>~/$ exiftool -xmp-dc:description="Foo" demo.png
      1 image files updated
  ~/$ exiftool -xmp-dc:mytest="Bar" demo.png
  Warning: Tag 'mytest' does not exist
  Nothing to do.
  </code></pre>