To add a Custom Form to a page, you will create the form under onMessage > Content > Custom Forms. Then, you will go to onMessage > Website, and to the right of the page you want to put the form on, click the gray wrench icon, and select Edit Page Content.

From the lower left, drag and drop the Custom Form Content Type on to your page, and select the Form you want to use. Save the changes, then the URL for that page will be used to access the Custom Form.