Conditional Rules: Make form field mandatory if no default value present

Hi All,

I am wondering if some one can help me build a conditional form field tag as i have checked the below link and couldnt figure it out.

https://helpx.adobe.com/sign/using/text-tag.html

I am trying to make the below field tag mandatory and editable only if there is no default value present (in opportunity field) and if there is a default value this field should be read only.

{{*BankAccountName_es_:default("<<Opportunity>>")}}

Thanks in advance.

Abu