Hi Yalçın,
Since you are managing the accounts, you can add these credentials in the
config file directly. You can also create your credentials to make the call against the account. Usually, to make an API call to the client's account, you need to have credentials of the user having access to the account or the credentials of manager account. To make the API call please refer to
this guide. Let me know if you have any further questions.
Thanks,
Milind, AdWords API Team