In user defined form, E-mail, add vendor's contact person' or vendor's email address should come

Hello experts,

I have created a form Purchase Inquiry (Document type). Now When I click on E-mail in recipients' list, vendor's email address or default contact person's email address should come.

How can I achieve this??

I tried adding email address in recipients' matrix , but it doesn't allow.

Right now, in recipients' lit creator of the document (UserSign)'s email address automatically comes in the list.

Like this, what can I do that vendor's email address comes automatically?

PFA..

Thanks in advance.