@Ryez Please do the following:
Open Steam Client
On the upper bar that has the Store and Library menu items click the one that is your account name
In the context menu that appears press Profile
In the profile window that now loads in the upper right is an "Edit Profile" button, press that
In the page you are now on is a TextBox with the label Custom Url, set that to whatever value you want
Return to LaunchBox and put the Custom URL value you just entered into the text box asking you for your Steam ID
Finish running the wizard (assuming your game list is already public)