Invoking particular action on dropdown list selection in MVC

Do you really need to submit the form? You could redirect:

onchange = "redirect(this.value)"

where redirect is a custom defined function:

function redirect(dropDownValue) {
    window.location.href = '/myController/myAction/' + dropDownValue;
}

Your approach should work. The issue you're encountering is your use of this in your onchange event is invalid. Try replacing your this.form references with something like this:

<%= Html.DropDownList(
        "ddl", ViewData["AvailableList"] as SelectList, 
        new { onchange = @"
            var form = document.forms[0]; 
            form.action='MyMethod';
            form.submit();" 
        } ) %>