Hi, I want to load custom record on PurchaseOrder creation I have the custom field of custom record and I would be able to get the id of custom record but I want to know which script do I need to use to load Header and Item Information in creation.

You need to explain this a little better. So a purchase order gets created. It has a custom field on it that gets populated with a custom field ID, right? What do you actually want to do with this related custom record? Once you've loaded the record, what sort of processing do you want to do? Does it require further modification of the Purchase Order?

@CD Thanks for you response, while creating Purchase Order I have a custom field called PO Req, which is in dropdown once I select the value from that dropdown i want to catch the event to load rest of the record from Purchase Requestion record and load it in the Purchase Order form. Please have a look at the attachment. Circled field values should loaded from custom record type PO Requestion.

You just need a simple client script that detects the field change on your dropdown, loads the related record, then populates the fields. Have you written any suitescript before?