// string with markups data generated with markupsExt.generateData()
var modelMarkups = '...';

// Load the markups extension
viewer.loadExtension('Autodesk.Viewing.MarkupsCore').then(function(markupsExt){

    markupsExt.show();

    // Load markups onto "layerA"
    markupsExt.loadMarkups(modelMarkups, "layerA");

     // Allow editing of markups in "layerA"
    markupsExt.enterEditMode("layerA");
})