Google ads conversion tracking - leaving to external site

Hey folks, I have a problem... I know (more or less) how to track woocommerce purchases, thank you page on a website, ect, generally page that is at a website/domain I run ads for. But now I have campaign where purchase is made on external site/app, completely different domain where I have no admin rights, no nothing. How do I measure conversions on that? Basically I want to count conversion when client leaves to specified external url. Any suggestions are welcome.

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Come on people