You put the URL into the first [URL] tag by putting and equal sign and your link in quotations within it. Normally the URL sits between the tags, but in this case you put it inside the beginning tag:
e.g. [URL="http://your.link.ca"]
Then put your text that you want displayed instead of the URL between the tags:
e.g. [ URL="your.link.ca" ]Text to display[ /URL ]
(ignore the spaces in the tag brackets...I had to put them there to make it show the text as the forum ends up using the actual tags and formats it)
So if you do it right, it should show as "Text to display" and it links to the URL (your BST thread for example) you specify:
Text to display