Question

How can we get DocumentID as a field?

Answer

1. Create a new Text Field "DocumentID" in your Document Definition.
2. In the Rules tab of the "DocumentID" field properties create a new Script Rule. 

Make sure to disable the Read-Only option for the "DocumentID" field(it is on by default) and enable the rule.

3. Add the following script to the rule:

this.Field("DocumentID").Value = this.Document.Id;

This will allow us to set the value of the specified field to be equal to the DocumentID value for the current document.