How to set value of custom column automatically?

Hi,

I have a question but I apologize if this is not the proper place to ask. If so, please kindly let me know where I should ask my question. Many thanks in advance.


I am trying to create Metadata Plugin for new Web page.
I succeeded in setting title , author and publisher automatically when I choose to download metadata.
But I can't understand how to set value of a custom column.

What would be the best way to do this?