So, while this is not exactly a typical "self-hosting" question as many users might not be using domains, I would be curious if anyone else has any experience with this.

I have NGinx Proxy Manager installed on a vps and a few docker instances that host various services (wordpress, a gitlab, etc etc) that I have bound to specific ports (wordpress to port 80, gitlab to port 3000, to give made up arbitrary examples.)

I also have a domain and a few subdomains registered as Type A resource records that look like:
[www.]somedomain[.com]
[gitlab.]somedomain[.com]

The essence of the question: When I go to NGinx Proxy Manager and register a "Proxy Host" for the gitlab subdomain, like:

Domain: gitlab.somedomain.com
Scheme: http
Forward Hostname: <IP ADDRESS HERE>
Forward Port: 3000 (AKA the port gitlab is hosted on)

This works, but it comes with the drawback that the port number is then exposed in the url bar like so:

gitlab.somedomain.com:3000

So is there some way to fix this on the NGINX proxy manager side of things? Or is this a case where I'm doing this completely wrong and someone with web-dev experience can help me see the light. While it's not a huge hindrance to my use-case, it would still be nice to understand how this is supposed to work so that I can host more services myself that require domain names without having to shell out for isolated IPs. So if I hosted a lemmy or kbin, for example, I could actually configure it to use my subdomains correctly.