You can follow the instructions provided
here to link your Manager account to the client account. You can also use AdWords API to do this as described
here. But in this case, you would need the OAuth credentials of the client account as well as the Manager account. Once your Manager account is linked to the client account being targeted in the request, you'll be able to use the OAuth credentials of your Manager account to authorize requests to the client account by specifying the
clientCustomerId in the
request header.