Hi Sachin,
Thank you for reaching out.
Before you can use the AdWords API, you have to generate your OAuth2 Credentials first. Could you confirm if you have already generated one? If not, you may refer to this
guide on how to do so.
After generating your credentials, you may now start making API calls on your test accounts, that includes addCampaigns.
Regards,
Dannison
Google Ads API Team

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