post https://eu-ext.linnworks.net/api/PurchaseOrder/Add_PurchaseOrderExtendedProperty
Adds extended properties to a purchase order. Property name must be unique otherwise the method throws an error. Maxium number of extended properties allowed for a single purchase order is 50. If total count of extended properties exceeds 50 an error will be thrown.
Permissions Required: GlobalPermissions.Inventory.PurchaseOrder.EditPurchaseOrderNode
Rate limit: 150 / minute