On the XPage or Custom Control, go to the document's data source Events tab.
Then select the postNewDocument event under data.
Then select the Add action to add a simple action for this event. From the dialog select All from Category and Set Value from Action.
Then add 'dominoDocument1.From' computed value for Binding in expression lanuage - (dominoDocument1 is your data source and From is your authors field in that Form from which the data source is set to )
And then add '@UserName()' as a computed value to Value