Using the OTBR AddOn (on HAOS) as the example, what generally happens, is the OTBR AddOn boots up and sends discovery information to the HA OTBR Integration.
From the OTBR AddOn Log:
[13:15:56] INFO: Successfully sent discovery information to Home Assistant.
s6-rc: info: service otbr-agent-rest-discovery successfully started
Through magical means through various HA Integrations, this is indeed hidden to the user, and the OTBR Integration is suppose to also auto-discover the URL without having to have user tell it.
This URL is what the OTBR Integration uses to talk to the OTBR AddOn’s REST API port. For the OTBR AddOn, this is normally:http://core-openthread-border-router:8081. If you goto the OTBR AddOn’s Info web page you’ll see there is a “hostname” core-openthread-border-router. It is this hostname that is used in the URL. BTW, if you are using multiprotocol (which is no longer recommended) it would be: core-silabs-multiprotocol:8081.
If your OTBR Integration is popping up a box asking you to supply a URL, it is probably because it didn’t auto-discover a URL, or whatever URL it has it tried to contact the OTBR but it failed. The main purpose of having to manually enter a URL is for cases where someone has rolled their own OTBR.