KaushaL-
To have it show up during the user signup process, you need to set your custom property to be "Required" and then enable two settings under ADMIN -> USER ACCOUNTS -> USER SETTINGS

The two setting to enable are:

1. Require valid profile during account creation.
    
2. Require valid profile during next login.

 
For #1, any profile fields marked as "required" will be presented to the user during the account registration process.  This means that existing users with accounts are not required to fill out the field(s) newly marked as "required".
 
For #2, an account can be created without all the "required" user profile fields (though "user credential" fields are still required).  However, upon the user's next login, the user will be prompted for the "required" profile fields (this is a little confusing).
 
So, to make any profile field "required" for both existing and new users, enable both #1 and #2.
 
Cheers!
-mamlin