NGinxPM subdomain proxy himself

[mauroreggio]

Hi, i run npm in docker. I think i can setup it in a good way and i apreciate how is made, thanks. Only i desire understend how proxy npm himself with subdomain.
Example:

• server.example.com --> proxyed to a grafana environment in a protected docker virtual network (and this work in a good way)
• server.example.com/npm --> proxyed to npm himself at port 81 in a protected docker network (and this not works)
  I try read many in internet , try "Custom Location" and try "Advanced Options".
  If i put in "Custom Location" i can see that the conf file of the proxyed host have the entry with "location" (/npm) and "server" (http://nginxpm:81) specified, but when i try surf:
• the page ramain blank
• in the log i can see that the request was redirect to the grafana server instead.
  Any suggest?
  Thanks.
  Mauro.

Edit: if i use second level domain, all works good, but i prefere not use this approach if is possible.
Example:

• server.example.com --> proxyed to a grafana environment in a protected docker virtual network (and this work in a good way)
• npm.server.example.com --> proxyed to npm himself at port 81 in a protected docker network (and this work in a good way)