To do copy and paste you need to invoke a script, so you need a script button to press. You can link your custom field to this script button so that typing an article and pressing "Enter" will invoke the script, rather than requiring the user to press the script button. Having built the link you can even hide the button.
Pasting the value might be more complicated, although it looks like you might be trying to create a one-line purchase order. If so, that makes life easy. Record a script where you type a value into the Article field of the first line of the table, and then change the "Enter Value" action to a "Paste Value" action and paste the value you copied. If you look at the cell references in that recording, you'll see how to address individual cells in a table, should you wish to paste into lines other than the first.
Does that help? If you need more info, please ask.
Steve.