insert element or block inside parent element

Good day, i have component, have block

editor.BlockManager.add("bootstrap-container", {
     attributes: {class: "gjs-fonts gjs-f-b1 gjs-block gjs-one-bg gjs-four-color-h"},
     category: "test",
     label: "Test",
     content: '<div class="container" style="min-height:10px;"></div>'
   });

i have Trait on listen changes, how can I add "h1 " inside that block "'<div class="container" style="min-height:10px;"></div>". How to add tag inside this model like:

doStuff(){
                 var coll = this.collection;
                 var at = coll.indexOf(this)
                  coll.add('<h1>Hello world</h1>', { at });
}