AWS DMS connect to on premise SQL Server DB
The network address of a server instance (its server network address or Endpoint URL) contains the port number of its endpoint, as well as the system and domain name of its host computer. The port number uniquely identifies a specific server instances.

You can try for example below format as we use for Database Mirroring :

TCP://srv5.corp.abc.com:5022

For your reference:


Configure SQL Server Database Mirroring Using SSMS
The Database Mirroring Endpoint (SQL Server)