For hiring contractors or service providers, purchaser is expected to issue Purchase Order (PO). Since you will be providing the services, company may issue the Work Order (WO) however, whether issued PO or WO, more or less the contents remain same. Ensure that payment terms are clearly specified.
You will get whole lot of format of POs from the internet. However, what matters is conditions of services. You need to design these on your own.
18th January 2017 From India, Bangalore