[kh99]

Here's the manual page on creating a plugin: https://www.vbulletin.com/docs/html/main/add_plugin (or at least the instructions for using the admincp - it doesn't say much about the coding part). You just need to add some php code in the large text area that sets $fromusername to what you want. If you want to see where it will be executed, look at includes/class_dm_pm.php and search for 'pmdata_postsave_recipient'.