I found the solution here: https://github.com/camunda/camunda-modeler/blob/master/client/lib/app/editor/bpmn-editor.js
First you have to require your own templates:
var templates = require('../resources/element-templates/templates.json');
When initializing your Modeler, don’t forget the element templates:
var renderer = new Modeler({
container: canvas,
keyboard: {bindTo: document},
additionalModules: [
propertiesPanelModule,
propertiesProviderModule
],
elementTemplates: templates,
moddleExtensions: {camunda: camundaModdleDescriptor},
propertiesPanel: {
parent: '#js-properties-panel'
}
});