Empty Field Annotation

Question

I am training the custom model (Template) in Azure Form recognizer. The scenario is
a form with 5 fields - first name, last name, age, gender, address, phone.
I have to train a model to detect all these fields, but last name is empty most of the time, how do I annotate the empty documents where last name is not present.
The result I want is to detect last name when it is present.

I have two options-:

1. Annotate last name only in documents where it is present.
2. In documents with empty last name, annotate it using a bounding box where it is supposed to be present.
3. Create some dummy documents with dummy last name and train on these documents.

What would be the best approach for my task?