To create measurement annotations in WebViewer, click the Measure button in the top header, then select one of the measurement tools. Next use the selected tool to create a measurement annotation on the document. An overlay with scale and measurement information will show up when you are creating or selecting a measurement annotation.

If there's no scale created before, the Scale modal will pop up when clicking on the document. Click the Create button to create a new scale. This initial scale will be applied to all the measurement tools.

The following example shows how you can determine if an annotation is a measurement annotation, and logs the scale and precision of it after it's added:

WebViewer(...)
  .then(instance => {
    const { annotationManager } = instance.Core;

    annotationManager.addEventListener('annotationChanged', (annotations, action) => {
      if (action === 'add') {
        // An annotation is an measurement annotation if it contains a Measure property
        const measurementAnnotations = annotations.filter(annotation => annotation.Measure);

        measurementAnnotations.forEach(annotation => {
          console.log(annotation.Scale);
          console.log(annotation.Precision);
        });
      }
    });
  });