gistfile1.js

<script>
var gm=$("#mycanvas").gridmanager({  
    // Add the custom button
		customControls: {
             global_col: [{ callback: 'insert_image', loc: 'top', iconClass: 'fa fa-file-image-o' }]
        }  
	}).data('gridmanager');

    // Function which gets called
	function insert_image(container, btnElem) {
	    // In this instance, I'm getting a remote AJAX HTML fragment to populate bootbox.js dialog
		$.ajax({url: "/admin/imagemanager/dialog",
			success: function(data){
			    // Bootbox modal call
				bootbox.dialog({
				  // Populate bootbox with data from ajax call
				  message: data,
				  title: "Image Manager",
				  buttons:{
					  success: {
					      label: "Insert Image",
	      				  className: "btn-success",
	      				  // This is th
						  callback: function(e){
						  	var selectedImage=$("#image-dialog").find(".image-dialog-selected");
						  		selectedImage.data({imageSize: $("#image-size").val()});
						  	var sc="[image src='"
						  	+ selectedImage.data("imageName")
						  	+ "'"
						  	+ " size='"
						  	+ $("#image-size").val()
						  	+ "' caption='"
						  	+ selectedImage.data("imageCaption")
						  	+ "']";
						  	// This bit is the key - it adds an editable region with the string I want to insert already in it
						  	gm.addEditableAreaClick(container, btnElem, sc);
						  }
					  }

				  }
				});
			}
		});
	}
	</script>