Hi Tanvir,
Thanks for taking the time to to submit this request. I see that Ionis do have a Domain API, however it only supports managing existing domain registrations in your account. It appears to be missing the key functions to register and transfer-in new domains or renew existing domains: https://developer.hosting.ionos.com/docs/domains

For this reason we must decline this request. If Ionis implement an API with all the required functionality, we can potentially re-consider under a new request in future.