Here is the serial section of the DOSBOX options file.
The # are just comments and are in the file by default.
The salient lines are the ones I have highlighted.
[serial]
# serial1: set type of device connected to com port.
# Can be disabled, dummy, modem, nullmodem, directserial.
# Additional parameters must be in the same line in the form of
# parameter:value. Parameter for all types is irq (optional).
# for directserial: realport (required), rxdelay (optional).
# (realport:COM1 realport:ttyS0).
# for modem: listenport (optional).
# for nullmodem: server, rxdelay, txdelay, telnet, usedtr,
# transparent, port, inhsocket (all optional).
# Example: serial1=modem listenport:5000
# Possible values: dummy, disabled, modem, nullmodem, directserial.
# serial2: see serial1
# Possible values: dummy, disabled, modem, nullmodem, directserial.
# serial3: see serial1
# Possible values: dummy, disabled, modem, nullmodem, directserial.
# serial4: see serial1
# Possible values: dummy, disabled, modem, nullmodem, directserial.
serial1=directserial realport:COM1
serial2=dummy
serial3=disabled
serial4=disabled
[autoexec]
mount c c:\psp
c: