Which version of Thundercats is your fanfic based on: the original 1980s version, or the recent Cartoon Network remake?

And even if you do not know HTML, all you need to do is copy and paste the URL; it will automatically be converted into a link.

This board uses HTML for post formatting.
Therefore, links are written (replace texts in [] as appropriate) <a href="[thing you're linking to]">[text of link]</a> For example, an example (view source to see how I did it).

As a sidenote, if you ever need to write literal angle brackets, you must use &lt; for < and &gt; for >. An & character is written &amp;.

I can't help you with your crossover, sadly, since I don't know either of the fandoms.