If you want the signature field to be optional, simply use the following format:
The third character “X” can be either a question mark (?) or an asterisk (*). An asterisk indicates that an Input Field or Signature is required. A question mark indicates that it is optional.
you can find the document used in the code from this link:
Refer to this document page for more information: https://docs.esignlive.com/content/c_integrator_s_guide/sdk/c_managing_documents/extraction.htm#TextTagExtraction