how to automatically click a html button when asp.net button is clicked

Attach the onclick event in the markup of the ASPX page (on the button):

onclick="#('elementName').click();"

Now, if you don't want the post back to occur on the ASP.NET button, then do this:

onclick="#('elementName').click(); return false;"

In the asp button, add a OnClientClick event to call a javascript function and then trigger the second button click from within.

<asp:Button Text="Server Button" runat="server" OnClientClick="js_function();"/>
<input type="button" id="inpbutton" value="Client Button"/>

function js_function()
{
    $("#inpbutton").trigger("click");
}

This should do it. Update the selectors with the ones in your page

<script>
    $('#<%=ASPBUTTON.ClientID %>').click(function(event)
    {
        $('OTHER BUTTON ELEMENT SELECTOR').click();
        event.preventDefault();
        return false;
    });
</script>

You can do this in two way.

first by using common class name for both button

<button name='cmd1' class='cmd' >button 1</button>
 <button name='cmd2' class='cmd' >button 2</button>

jQuery

//common click event for both button
$('.cmd').bind('click', function(){
  ...
  ...
});

and for second method you can create two seprate onclick function for both button , when a button get click (source) inside that function call target function , like this

<button name='cmd1' onclick='first_function()' >button 1</button>
<button name='cmd2' onclick='second_function()' >button 2</button>

Javascript

function first_function(){
     // call second function
     second_function();
     ...
     ...
    }