FreeTube is supported by the Privacy Redirect and LibRedirect extensions, which will allow you to open YouTube links into FreeTube. You must enable the option within the advanced settings of the extension for it to work.
If you have issues with the extension working with FreeTube, please create an issue in the FreeTube Repository instead of the extension repository. Most if not all problems is a problem with FreeTube itself and not a problem with the extension.
This extension does not work on Linux portable builds!