To create a new signature field with required flag and a signature widget annotation.

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

  // myBtn is your own custom button
  document.getElementById('myBtn').addEventListener('click', () => {

    // set flags for required
    const flags = new WidgetFlags();
    flags.set('Required', true);

    // create a form field
    const field = new Annotations.Forms.Field("some signature field name", { 
      type: 'Sig', 
      flags,
    });

    // create a widget annotation
    const widgetAnnot = new Annotations.SignatureWidgetAnnotation(field, {
      appearance: '_DEFAULT',
      appearances: {
        _DEFAULT: {
          Normal: {
            data:
              'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAAYdEVYdFNvZnR3YXJlAHBhaW50Lm5ldCA0LjEuMWMqnEsAAAANSURBVBhXY/j//z8DAAj8Av6IXwbgAAAAAElFTkSuQmCC',
            offset: {
              x: 100,
              y: 100,
            },
          },
        },
      },
    });

    // set position and size
    widgetAnnot.PageNumber = 1;
    widgetAnnot.X = 100;
    widgetAnnot.Y = 100;
    widgetAnnot.Width = 50;
    widgetAnnot.Height = 20;

    //add the form field and widget annotation
    annotationManager.getFieldManager().addField(field);
    annotationManager.addAnnotation(widgetAnnot);
    annotationManager.drawAnnotationsFromList([widgetAnnot]);
  });
});

Create fillable PDF forms
Full sample code demonstrating how to add form fields to a PDF. Fields are first added as annotations and then converted to interactive form fields. Add signatures, text fields, checkboxes.

Available widget flags are:

• ReadOnly
• Required
• NoExport
• Multiline
• Password
• NoToggleToOff
• Radio
• PushButton
• Combo
• Edit
• DoNotScroll
• RadiosInUnison