In ASP.NET if you want to associate code behind with a submit button you should use ASP:BUTTON control like in the next example:
<asp:Button ID="saveButton" Text='Save' runat="server" OnClick="saveButton_Click"/>
Then in your code you should implement the method linked with the button above:
protected void saveButton_Click(object sender, EventArgs e)
{
}
Note that this event handler for your button click will be executed after the
Page_Load
event.