There's a control <select> on your page and you want to use it's value as selected and don't want to allow user to change it. Unfortunately, there's no attribute "readonly" for select as for <input> ones. You can use only attribute "disabled", but in this case its value will not be sent. But there's a trick how to do it.

First you should add attribute "disabled" and set it to "true". In this case after sending your form the value will not be sent to a server. But you can change the value of attribute before sending the form! You should just set the value of attribute "disabled" to false.

For example:

<!-- This is html code of the form -->
<select name="tt" title="Status">
<option value="No">No</option>
<option value="Yes">Yes</option>
<option value="May be">May be</option>
</select>
 <br />
<input type="Submit" id="sendbtn" value="Send">


<!-- This is javascript -->
<script type="text/javascript" src="node_modules/jquery/dist/jquery.min.js"></script>
<script type="text/javascript">
    $(function() {
        $statusCtrl = $("select[title^='Status']"); 
        $statusCtrl.val('Yes'); 
        $statusCtrl.attr("disabled", true); 

        $('#sendbtn').click(function() {
            $statusCtrl.attr("disabled", false);             
        });
    });
</script>

So, you should change attribute "disabled" value just before submitting.

For example, for SharePoint forms it you should override function OnPreSaveItem().