Enter Key submition is not raising the command event

[Jitu]

<form runat=server id=myform>
<AT_COMMON:SitePage id="DefaultPage" runAt="Server">
<itemTemplate>
<AT:UsersRegistration id="Registration" RunAt="Server"
UserFormName="Users/Users/UserForm.ascx"
SkinFileName="Users/Users/UserRegistration.ascx" />
</itemTemplate>
</AT_COMMON:SitePage>
</form>

Above is the hirarcy of my controls in the form.

1] In the control AT:UsersRegistration there is one control again
UserForm.
2] The AT:UsersRegistration has submit ('_submit') button and cancel
('_cancel') button.
3] I want to fire the _submit_command event while user submits form
using enter key. The Form gets submitted but, the event is not fired.
4] I've registered default command using
Page.RegisterHiddenField("__EVENTTARGET", "_submit")

Now See the difference in below hirarchy.

<form runat=server id=myform>
<AT:UsersRegistration id="Registration" RunAt="Server"
UserFormName="Users/Users/UserForm.ascx"
SkinFileName="Users/Users/UserRegistration.ascx" />
</itemTemplate>
</form>

1] the event gets fired in enter key submition.

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Can anyone help me on this?

Thank you.

Jitu