Benito
You could use the pages extension (lets you create static pages) and the links extension (lets you add links in the header like a main menu). Then, for the contact form itself you can use any free PHP contact form script out there. Then, include the contact form in the Flarum page via an iFrame. If you do it like this, with this CSS, the height will adjust to your content.
<div style="position:relative;height:100%;">
<iframe src="https://yoursite.com/path/to/form/" style="height:100%;width:100%; border:none"></iframe>
</div>