Promised date to be default when a new item line is created

Hi,

I have a urgent requirement to make the default promised date in Purchase order form

Promised date on PO Lines- Whenever an item is entered in PO line,  check the “FULL_LEAD_TIME” from Item master for that item. Add the number of days in Full lead time field to Sys date and populate the date in Promised Date. User can override the defaulted value, if needed. The Organization for the item is to be obtained from the organization attached to the Ship to Location.

Can you please help how we can do that?

Thanks,

Suvendu