Register on the sandbox portal for one of the GSTINs of the Company and test the APIs by changing the internal ERP/accounting system.
Login to the einvoice1.gst.gov.in portal using a web username. If it is not there, register on the portal and create a web username.
Select API registration and ‘Create API user’. Here, select the GSP and GSP names through which access is required. Then create your username and password and preserve them.
Next, use the client id and secret, along with the username and password, and access the APIs and generate the IRNs.
Here, it may note that you may not need to whitelist the IPs and submit the test summary report.
Note that one can create a separate username and password for a separate GSP (Client) for e-invoice generation. However, corresponding username credentials and client credentials are used for API access and IRN generation.