$(function(){
$('#boutonCreerPlat').on('click', function(){
    $.ajax({
        type: "POST",
        url: '',
        data: 'string',
        success: $.post( "plat.php", function( data ) {
            $( "#plat" ).append( data );
        }),
        dataType: 'string'
    });
});


<?php
echo '<div class="form-group">
        <label for="exampleInputFile">Créer un plat</label>
        <input name="nomPlat" type="text" class="form-control">
        <select name="prodSelect"></select>
        <button class="btn btn-secondary">Ajouter un produit +</button>
     </div>';