Normally you can open reservation via reservation button next to theme’s main menu. However you can also link from other elements too. Here are steps.

For example you have this button code

<a href="#" class="custom_reservation button">Your Text</a>

Then you need to add this elements class name to reservation javascript. Open /wp-content/themes/grandrestaurant/js/custom.js at line 900 change from

jQuery('#tg_reservation').on( 'click', function() {

to this code.

jQuery('#tg_reservation, .custom_reservation’).on( 'click', function() {