Server address possible without port specification?

Hello, is it possible to specify the server address in the browser app without a port?

We have a firewall at work that does not allow port specifications.

I created a subdomain with my web provider and let it point to the DynDNS address of my Synology.

I have entered the following in the reverse proxy on the Synology:

Protocol: https
Port: 443

Protocol: http
Hostname: localhost
Port: 5151

It seems to me that something is still missing, because unfortunately it doesn’t work like that.
I get the following error message in the Chrome app:
Unexpected token <in JSON at position 0

Does anyone have any idea what I’m doing wrong?

Thank you very much!

Regards, Thomas

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