Add user method API

Hi,
 
While creating an user in HC method we basically called is AddUser and I am sending also and optional parameter for Enable Auto Billing so that i could get the Next Billing Date later on. But whenever i call this API method for Enable Billing  there is no effect. To confirm this i did changes through HC Panel through Billiing System :: Config Setting :: Enable Invoice, under this when i change to enable auto billing it start working. Could you please help me that what is the issue or do i have to make these changes through HC Panel. Currently i using HC 8
 
Thanks