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Go to your child template and select “edit template files” (from the list click on the icon that looks like a page).
While on that page select from the 1st. drop-down “Pages”, and from the 2nd. drop-down “account/registration”.
Then add the component clicking on the “+ Choose” > Select “Captcha Component”. Then add the component to the markup.
Check the image for reference:
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Hope that helps.