Right way to use Fusion form/UriBuilder

Currently I’m trying to implement a fusion contact form which sends the conclusion over mail.

The problem is I really don’t get how to configure the target UriBuilder.
I always get an empty action string.

I’m using the example form from the Neos Documentation.

What i already tried is to write a Controller as following:

Vendor.Site.ContactController.sendContact = Neos.Fusion:Component {
    renderer = afx`
        Test
    `
}

and the form target like:

form.target.action="sendContact"
form.target.controller="ContactController"
form.target.package="Vender.Site"

Would be perfekt if someone could provide me a running example.

Thanks in advance