ok i found answer myself :
add these namespaces:
using yaf;
using yaf.pages;
protected void CreateForumUserAccount()
{
ForumPage forumPage = new ForumPage();
int PageBoardID = 1;
string UserName = UserNameTextBox.Text;
string Password = PasswordTextBox.Text;
string Email = UserNameTextBox.Text;
string Location = "No location specified";
string HomePage = "http://www.anywebsite.com";
object timeZone = "-300"; // Eastern Time (US & Canada), Bogota, Lima, Quito
DB.user_register(forumPage,
PageBoardID,
UserName,
Password,
Email,
Location,
HomePage,
timeZone,
forumPage.BoardSettings.EmailVerification);
}