Hello Dogan,
There are two different ways to get access on a account as mentioned below:
- The client should give access to your email address or in other words you should be having access to the clients(or its high level manager's) account to make API calls. This can be done via UI only as mentioned here.
- The other way is the Admin user of the client's account should authorize your OAuth credentials while creating the OAuth credential. In the second way you will only be having access to the client account via API only and will be having access level of the user that authorized your credentials.
Please let me know if you have any further questions.
Regards,
Sai Teja, Google Ads API Team

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