Android Question WebView don't show scriptmsg


Hi everyone,
my colleague wrote a webapp with visual studio that allor the users to login to save data on a database and show a confirmation message, and it works correctly via the browser.
I've write an app with B4A that just load this webpage in a webview so i can easily share the apk with other users.
If i use my application the webapp is load correctly, but when i press for save data, the data are saved but the confirmation message doesn't show.

I don't know if i'm trying to do something completly wrong and i have to recreate the entire app in B4A or i just miss some code that allow display the message

The message, in the webapp, is show with
ScriptManager.RegisterStartupScript(Me, Me.GetType(), "abcdef", "alert('" & sMessage & "')", True)
My code is just:
B4A App:
Sub Activity_Resume
    If WebView.IsInitialized=False Then
        Activity.AddView(WebView, 0, 0, 100%x, 100%y)
    End If
End Sub