Adding details to the Notes tab on Sent and Closed purchase orders

The Notes tab is disabled when the Status of the purchase order is changed from Not Sent.  How can Notes be added in this situation?

Step-by-step guide

 in 6.5.13 there were system settings that controlled that. In Version 7 this is now controlled via a standard plugin - Allow Notes in Purchase Orders To Always Be Editable. Enable that and Notes will then be able to be edited and added on Sent and Closed purchase orders.

  1. System Settings > Plugins > Plugin Maintenance

  2. Load the search window and select Allow Notes in Purchase Orders to Always be Editable

  3. Tick Enabled and save

  4. Log out 

  5. Log back in again to compile the plugin

  6. Notes can now be added to purchase orders regardless of the status of the form