How I can get AssetManger selected image from outside the editor

I found the solution

function getGrapeJsAssetManager(editor,targetElem,targetInput){
  editor.runCommand('open-assets', {
              target: targetElem,
              onSelect: function onSelect(t) {
				targetElem.attr('src',t.id);
				targetInput.val(t.id);
              },
              onClick: function onSelect(t) {
				targetElem.attr('src',t.id);
				targetInput.val(t.id);
              }
            });
}

It helps me to use asset manager for changing the image field outside the editor. Is this is the right way to use it?

Yes, but probably I'd remove onClick function as it's already done by onSelect