You can access and fill any form field by its field id:

// config.js
const { documentViewer, annotationManager } = instance.Core;

documentViewer.addEventListener('documentLoaded', () => {
  documentViewer.getAnnotationsLoadedPromise().then(() => {
    const fieldManager = annotationManager.getFieldManager();
    const field = fieldManager.getField(fieldId);
    field.setValue('new value');
  });
});

You can also iterate over all fields using the forEachField function of FieldManager.

// config.js
const { documentViewer, annotationManager } = instance.Core;

documentViewer.addEventListener('documentLoaded', () => {
  documentViewer.getAnnotationsLoadedPromise().then(() => {
    // iterate over fields
    const fieldManager = annotationManager.getFieldManager();
    fieldManager.forEachField(field => {
      console.log(field.getValue());
      field.setValue('new value');
    });
  });
});

The fieldChanged event is fired any time the value of a form field changes.

// config.js
const { annotationManager } = instance.Core;

annotationManager.addEventListener('fieldChanged', (field, value) => {
  console.log(`Field changed: ${field.name}, ${value}`);
});