You can use forms in your microsites - but only in 'advanced mode'.
Whenever you create a new page for your microsite, there is an Advanced mode checkbox that can be selected on the 'Page properties' screen:
If you select advanced mode, you will be able to edit your page's HTML in source mode. This option is generally only recommended for very advanced users. Once your page has been created, you cannot change this option. Additionally, no reporting data will be gathered for your page.