Currently, the youtube link feature on profiles only works if your channel has a legacy username URL, as the link will always be youtube.com/user/*whatever's entered in the box*. This makes it outdated, as only channels created before a certain point in time, I think that point in time is when google+ was integrated into the comments section in november 2013, as the channel I had before then had a legacy URL, but when I linked my account to google+ to keep the ability to post comments, I had a new channel created under my email display name, and that one didn't have a legacy URL automatically, so I had to make it as a custom one. Sometime since then, legacy URLs have been made completely inaccessible to channels that didn't already have one it seems.
More info about channel URLs:
https://support.google.com/youtube/answer/6180214?hl=en-GBThe channel I currently use was created in 2015, by then 500 subs was a requirement for getting a custom URL, which has since been lowered to 100 w/ other requirements. If you get a custom URL, you will have two URLs or your channel, assuming you created it too late to get a legacy user URL: ID-based URL (youtube.com/channel/*user ID*) and customised URL (youtube.com/c/*channel name + anything else you want to add). It would be nice to have this updated, so those whose channels do not have a legacy URL can have their channels linked on their profiles w/o having to use their sig.