Hello,
One way to do that is to add a checkbox field "Send Order" in your table, and to use this field as the trigger for the automation:

You add a "conditional logic" action set on field "Generate Purchase Order". If value is "PO Created" then:
- Send mail
- Update the record that triggered the automation to reset the checkbox and update a "Send Order status" field with value "Sent"
If "Generate Purchase Order" is not "PO Created" then update the record that triggered the automation to reset the checkbox and tu update field "Send order status" to "Not Sent".
You then have to options for your interface.
Option 1
If you use a list to display the different orders, then add the checkbox in the list and it becomes the button to send the order
Option 2
If you use an interface where you can have a Button with the "Update record" option, such as a "record review" interface to display a specific order on demand, you can add a button to update the checkbox to true and hence trigger the automation.
Regards,
Pascal