Visit https://www.billiving.com/Signup in your browser.
Fill out the form. Use the same email address as you have used for your PatientDoc account.
Please confirm your email address by clicking on a link received on your email address from Billiving.
Sign in your Billiving account.
Click on Settings button.
Under Setting button, click on button API integration.
Copy the API key from here.
Sign into your PatientDoc account. Click on Settings tab.
Click on Invoicing.
Paste your Billiving API Key here.