How to call external webservice on Purchase Order save button?

Summary:

While saving PO, we want to call API which will attach document to the purchase order.

Content (please ensure you mask any confidential information):

While saving PO, we want to call API which will attach document to the purchase order.

Version (include the version you are using, if applicable):

24C

Code Snippet (add any code snippets that support your topic, if applicable):