Signup issue

When someone wants to signup on one of my users blogs, they getting redirected to the main site instead to signup on a user blog.

Now its getting redirected to: https://www.domainname.com/wp-signup.php

but what i want is that users can signup on the user blog, like this: https://www.user.domainname.com/wp-signup.php

With login its all fine but with signup its getting redirected to the maindomain.

Anyone knows how to solve this?

thanks