This is an excerpt from the SPS documentation, located here:
Users have the option to encode the address of the destination server in their username, in the username field of their client application. Note that SPS automatically displays a login screen if it cannot determine the username used in the connection, or you have not encoded a destination server in the username field. You can specify the destination address in the login screen when prompted.
When encoding the address of the destination server in the username, there are a few points to keep in mind. Since most RDP client applications limit which special characters can be used in usernames, this is not always intuitive. For the Microsoft Remote Desktop application (mstsc) and the login screen that SPS displays, note the following points:
Use % character to separate the fields, for example: username%my-targetserver
To specify the port number of the server (if it does not use the default port), use the caret ^ character, for example: username%my-targetserver^6464
To specify an IPv6 address, replace the colons with carets, and enclose the address in parentheses. For example, to target the ::1 IP address, use username%(^^1). To target port 6464 of the same server, use username%(^^1)^6464.