I want to integrate my accounting software with the e-invoicing portal using API. Where can I find the necessary documentation?

For eWay Bill GSP Registration, 
 Step : 1 Open your Internet browser and login to eWay Bill Portal - https://ewaybillgst.gov.in. 
 Step: 2 In E-Way Bill System Login, enter details such as: 
 User Name - Enter eWay bill system user name. 
 Password - Enter eWay bill system password. 
 Enter Captcha - Enter the captcha as displayed on the screen. 
 Step:3 After entering the above details, click on Login button. 
 Step:4 After you login into eWay bill system, go to Registration > For GSP. Register your GSP Suvidha Provider form will open. 
 Step :5 Click on Send OTP button, a one-time password will be sent to your registered mobile number. 
 Step:6 In Enter OTP, enter the one-time password received on your registered mobile number and click on Verify OTP button. 
 Step:7 Register your GST Suvidha Provider screen will appear. Here, click on Add/New radio button. 
 Step:8 Enter all the details here such as: 
 GSP Name - Select Webtel Electronic Pvt Ltd from the list displayed. 
 User Name - You can enter suffix of 3 characters. (For ex. 'cvl') 
 Re-Enter User Name - Enter the user-name again. 
 Password - Enter your 15 characters password. 
 Re-Enter Password - Enter the password again. 
 Then Open BUSY Login The Company 
 Step:9 Administration --> Configuration --> Features / Options -->GST/VAT TAB 
 Step:10 Enable Einvoice Requried then Click on Config option then Add Place & Station 
 Step-11 Administration --> Configuration --> Voucher Series Configuration ->Sale ->Select Voucher Series ->Voucher Configuration 
 Step : 12 Enable Option Generate Einvoice After Saveing 
 Step : 13 After Feeding Sale Bill Click on Save and in Transport Detailes Tab Y Einvoice Reqd option