Here is an example of someone who has posted a link to another YouTube video in a comment: https://www.youtube.com/watch?v=8kPyrq6SEL0&lc=z13qe524mnnzzzl5f04cdnrhgy3vi1zjk50I tried to post a link to a video, but when I view the comment in a private browser session, it's hidden. 
How can I post links to YouTube videos in YouTube comments?
youtube
By default, comments with links are held for approval. The video owner must approve your comment in order for it to be shown.https://support.google.com/youtube/answer/4409780?ref_topic=6011167Held for review tab: Review comments that are held for approval based on your comment settings. You can approve or deny these comment to be published, either individually or in bulk.Marked as spam tab: Review comments that are marked as spam. You can approve or deny these comment to be published, either individually or in bulk.