How to write URS for automation testing tool?

Hello All,
In my company, we are planning to convert manual testing process into automation testing process. I belong to Drug Safety and Pharmacovigilance dept in Pharmaceutical industry. We are trying to purchase automation testing tool. In order to approach to vendor, i am writing URS for Automation testing tool. Can anyone please guide me what important points should be in URS for Automation testing tool?

Thanks in advance
Bharat