A channel link and signature are a little different - though both are allowed on the forums. If you would like to showcase your YouTube channel at the end of your posts all you need to do is add your channel ID (the random number and letter part of your YouTube URL) into the personal details section of your forum account
Remember when you copy this over you should only do the channel ID so for example if my channel URL was
https://www.youtube.com/channel/thisisatest so my channel ID would be
thisisatest , hope this helps explain that a bit better.
As for the signature you can also add this into the signature section of your forum account, though if you wanted one with has your subscriber / view goals that is automatically updated when these numbers increase you could try "Cyan Labs YouTube signature generator" as they take your profile banner from your YouTube account, let you write out the goal you wish to set and keep track of your progress, like you can see with mine below this post. Once you have used it as well all you need to do is copy and paste the URL they generate into the "signature" section of your forum account, save the changes and then it should be updated for you
Hope this helps, if you need anything else or have any further queries please don't hesitate to get in touch