It does not really matter where it is registered, as long as the registry is publicly available (most registries follow similar formats after all).

It may depend on the country in which the trial is conducted, e.g. in the Netherlands the Netherlands Trial Registry (NTR) is often used. A good international alternative is clinicaltrials.gov (free of charge). Best of luck!

@victor What are common registries?

clinicaltrials.gov

isrctn.com

clinicaltrialsregister.eu

Registries per country are listed under this link: https://www.hhs.gov/ohrp/international/clinical-trial-registries/index.html

@victor thanks