Open JDownloader
Go to Settings
Open Account Manager
Add or select your YouTube account
Open the browser where you are logged into YouTube
Export or copy the YouTube cookies from the browser using a cookie export extension or tool
Save the cookies as a compatible cookie file if required
In JDownloader, use the cookie import option for the YouTube account
Select the exported cookie file
Confirm the import
Refresh or recheck the account status in JDownloader
Restart JDownloader if the cookies do not apply immediately