Making an internal site available externally has nothing to do specifically with NGiNX or Process Automation, but rather configuring this with your DNS server. You need a domain that points to a name server that has a record for your site, and that site has to point to the NGiNX site. The external IP for NGiNX has to match the external DNS record. Firewalls will need to be properly configured as well.
This is something you will need to discuss this with your networking team.