I've made a webform on my own domain, outside the domain of my company. The form is made for employees of the company. When an employee wants to get to the form he has to login to the company website and click on a link. In this manner the employee doesn't need a special login for the form but he can use the standard company login. In the .htaccess on the server of the webform I check the HTTP_REFER. When the refer is correct access is granted and a cookie is set. When the employee accesses the form again he can go directly because he has a cookie.
<If "%{HTTP_REFERER} == 'https://sites.google.com/a/company/form'">
Header append Set-Cookie "verified_user=yes;path=/;Secure;HttpOnly;Expires=Wed, Jan 01 2020 2:02:02 GMT"
</If>
<Elseif "%{HTTP_COOKIE} != 'verified_user=yes'">
Redirect https://sites.google.com/a/company/form
</Elseif>
How secure is this ?