Another site is already using this domain#

I am trying to use this custom domain and connect it to https://tinkerer.netlify.app/
Custom Domain:
https://tinkererslabict.org
We had linked our custom domain to a netlify website on the developer’s account, but now we are unable to contact the developer to take that website down
the new website we want to link to our is https://tinkerer.netlify.app/

I click “Domain Settings” > “Add custom domain” > Enter domain and click “Verify”

https://tinkererslabict.org is already registered. If you registered this domain name through a different registrar, select Add domain to begin delegating it to Netlify.”

click “Add domain”

“Another site is already using this domain”

“Domains must be unique across Netlify. Currently another site is using this domain. If you don’t know which site, please contact support”

Please advise on how I should proceed
Thanks

You need to follow this guide to verify ownership. Once done, Netlify support can release the domain from the other account.