<form id="mr_project_form" class="post" role="form">
    <label class="fre-field-title" for="fre-project-title">Your project title</label>
    <input class="input-item text-field" id="fre-project-title" type="text" name="post_title" placeholder="Add more than 15 characters"/>
    <!--Submit button-->
</form>

$( "#mr_project_form" ).validate();

<form id="mr_project_form" class="post" role="form">
    <label class="fre-field-title" for="fre-project-title">Your project title</label>
    <input class="input-item text-field" id="fre-project-title" type="text" name="post_title" placeholder="Add more than 15 characters"/>

    <!--Inserted input-->
    <input class="input-item text-field" id="inserted_input" type="text" name="inserted_input"/>
    <!--Submit button-->
</form>

$.ajax({
            url: mr_ajax_url,
            type: "POST",
            data: dataString,
            success: function (response) {
                $("#"+insertLcation).html(response);

                $('#inserted_input').rules('add', {digit: true,});
            },
        });