Redmine

This should be a simple plugin witch adds a Button in the project menu with a Link defined in a custom project field:

The Link shout be opened in a new tab.

Since my lack of ruby knowledge I can't get it to work:

custom project field:
name: url_planliste; id: 1; Format: Link

init.rb:

Redmine::Plugin.register :planliste do

 menu(:project_menu,
       :Planliste,
       "http://xyz.xx/",   //here should be the value of the custom field
       :caption => Proc.new { "Planliste" })
     permission :caption, :planliste => :index  
end

Maybe someone could give me a hint to get it to work..

Thanks in advance,
Andreas Planegger