Just by reading the link you provided, it seems that you can only install it in the configuration section:
The value of the FormsCookieName property is set in the ASP.NET application configuration file using the name attribute of the form configuration element.
<authentication mode="Forms">
<forms loginUrl="member_login.aspx"
cookieless="UseCookies"
name=".ASPXFORMSAUTH" />
</authentication>
Also, does this property allow you to "get" the name correctly?
So, I guess the answer is No.