The first step is to Apply for GBP API

https://docs.google.com/forms/d/e/1FAIpQLSfC_FKSWzbSae_5rOpgwFeIUzXUF1JCQnlsZM_gC1I2UHjA3w/viewform

You need to enter a Project Id and Project Number

Enter all Details of your Agency

In Your Email section enter the same email that you were using while creating Project in Google Cloud

Choose Third-party / Reseller / Agency

there is a small bug there is no 5th page

For the section, Briefly describe your company’s business model and how you plan to use the Business Profile APIs Re-write the answer DO NOT COPY AND PASTE

Enter accurate information that represents your business

You MUST be approved by Google to complete the entire process. Please submit a screenshot of the email stating you were approved to us. The link below shows the email we need to see. If you do not have this email, you have not gotten approved yet.