Dropship Purchase Orders can only be created by manually clicking the Drop Ship or Speclink on the line item of a Sales Order. A sample code snippet for creating it in SuiteScript 2.0 is provided below.
var purchaseOrder = record.create ({
type: record.Type.PURCHASE_ORDER,
isDynamic: true,
defaultValues: {
recordmode: 'dynamic',
soid: '11111',
dropship: true,
custid: '22222',
entity: '33333'
}
})