Hi,
Thank you for reaching out to the Google Ads API support team.
I would recommend you refer to the document
Linking to Manager Accounts and the help center article
About linking accounts to your manager account to get more information on how to link the accounts to the manager account. Also, kindly note that it is not necessary that the individual account should be a child account of a manager account.
I hope this helps! Feel free to get back to us in case of any further queries.
