Mui_welcomepage_link
When you want to show a web link on the finish page you can use MUI_FINISHPAGE_LINK. Unfortunately, there isn't easy support to show link on the welcome page :(
So I decided to find out how to solve this problem:
In your nsi file define following defines, set appropriate values for: MUI_WELCOMEPAGE_LINK, MUI_WELCOMEPAGE_LINK_LOCATION
!define MUI_WELCOMEPAGE_LINK "web page"
!define MUI_WELCOMEPAGE_LINK_LOCATION "http://something"
!define MUI_WELCOMEPAGE_LINK_COLOR "000080"
upload my System.nsh and overwrite c:\Program Files\NSIS\Contrib\Modern UI\System.nsh.
Enjoy,
Michal