BBCode to make i linking download button - help me please

[Blackbeard]

[u*l=http://www.myplace.co.uk/forums/HDD.rar] [i*g]http://www.myplace.co.uk/forums/statusicon/forum_download.gif[/i*g] [/u*l]

how can turn this into a bbcode addon in messagebox, so when they click it it will ask them for the url of a file to download which is hosted on my servers and when posted it will display a gif download button which when clicked will start the download of the file.

i am wanting to host and post drivers and freeware/shareware pc maint files for my pc newbie forum.