There are actually two ways you can do this on Dr. Bill.
Through the mobile app
You can create a "No Charge Referral" by creating a claim and putting 03333 as the billing code, the physician you enter in the referring physician line will be the physician you are referring to.
You then complete the rest of the claim as normal and save it.
Through the web app
You can also do this through the web app by clicking on "patients", finding the patient you'll be referring and click "refer patient."
A window will pop up where you'll just need to fill out the details and where it says "refer to," you can enter in the referring physician you're referring to. Here's a short video that shows you exactly how: