Citrix MetaFrame Application for Windows 1.8 Administrator's Manual page 60

You cannot configure a modem or serial port as both a RAS service port
and a connection port.
You cannot configure a serial null modem cable connection using the Dial-Up
Networking Serial Cable between 2 PCs option. You must configure the
connection directly from Citrix Connection Configuration.
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To create an asynchronous ICA connection
1. Run Citrix Connection Configuration.
2. On the Connection menu, click New. The New Connection dialog box
appears.
3. Enter a name for the new connection and click Citrix ICA 3.0 in the Type list.
4. In the Transport list, click async. The New Connection dialog box shows
the configuration options for an asynchronous connection:
5. In the Device list, click the modem or COM port for this connection. To install
a modem, click Install Modem. The Install New Modem wizard guides you
through the process of installing and configuring a new modem.
6. Click OK.