.
Or you can write:
[ url=http://www.web address the long gobblygook ][ color=blue ][ u ] Your own text here [ /u ][ /color ][ /url ]
^ I've put spaces in so you can read the code. Up to you if you want colour and underscore.
All the reader would see is "Your own text here", underscored, in colour, and it would be a live link.