Select the "Reply" feature and make sure it's UBBCode enabled. In the post options immediately above the typing area is a line of small icons in squares..the first one is for the hyper link. Click on that and then just follow the directions. Make sure you've copied the address to which you want to link then paste it in the appropriate spot. Once you've done that and clicked on "OK" you'll get a new box where you can put in whatever it is you want to call the link.
I don't know if that's all clear but it's about as clear as I can make it without knowing the technical language
I'm something of a dolt when it comes to these things.