First, make sure to document your possession of the idea, prior to contacting InnoGames. If nothing else, show it to someone who is considered trustworthy by others, as well as by you. Use email instead of the contact form. An email can contain more samples of your work, increasing the likelihood that they'll take it seriously. It also creates an electronic trail at your end, as well as theirs. The address is found on the same page as the form.
Be aware, though, that a lot of creative businesses won't even look at unsolicited ideas, especially not undeveloped or generic ones, as there's a risk that it bears a resemblance to their existing ideas or projects, or to ideas/projects that are natural consequences of existing projects. If such ideas are brought to the creative department, it will make it more difficult to prove that they developed the idea independently, in case of a lawsuit. The more developed your idea is, the less likelihood that it will match anything they are already working on, and the more likely that it will be brought to someone in the creative department.
For these reasons, the idea should be well developed. At a minimum, they'll want to know how the UI will look and function, the storyline/progression through the game, and ideas for monetization. Put as much work into it as an author would put into a manuscript before sending it to a publisher or movie studio.
