Override Forms Authentication default domain attribute value programmatically

I have a project which is used for different domains. I want to know if it is possible to set the attribute domain below programmatically.
<forms name="name" the "domain" attribute value should be overridden programmatically in code, but not in web.config file. </forms>  So it meets the requirements below: * The built-in FormsAuthenticationModule uses the new domain value to renew each forms auth session, for domains mentioned below * Applications with different domains can use one domain, other application with different domains use different domain. The domain can be completely different, not just subdomain.
Example of domains that share cookie could be:
Set A
a.domain.com a.x.domain.com a.y.domain.com Set B
b.domain.com b.x.domain.com b.y.domain.com < ...