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Please use the args attribute of the <node> tag. To assist you better, I made a sample code. Please see below:

I created my_args.launch file as shown below:

ravi@dell:~/ros_ws/src/ros_tutorials/rospy_tutorials/001_talker_listener$ cat my_args.launch 
<launch>
    <node pkg="rospy_tutorials" type="listener.py" name="listener" args="/dev/ttyUSB0" output="screen" />
</launch>

I edited the listener.py so that it can capture sys.argv[1] as shown below:

#!/usr/bin/env python
# license removed for brevity
import sys
import rospy
from std_msgs.msg import String

def callback(data):
    rospy.loginfo(rospy.get_caller_id() + 'I heard %s', data.data)

def listener(param):
    rospy.init_node('listener', anonymous=True)
    rospy.loginfo("param:" + param)
    rospy.Subscriber('chatter', String, callback)
    rospy.spin()

if __name__ == '__main__':
    listener(sys.argv[1])

Finally, I ran the launch file. I can see the value in the terminal. Please see below:

$ roslaunch rospy_tutorials my_args.launch 
... logging to /home/ravi/.ros/log/828983fa-2e9e-11ed-bbf8-d707e2da4a66/roslaunch-dell-90235.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://dell:33681/

SUMMARY
========

PARAMETERS
 * /rosdistro: noetic
 * /rosversion: 1.15.14

NODES
  /
    listener (rospy_tutorials/listener.py)

ROS_MASTER_URI=http://localhost:11311

process[listener-1]: started with pid [90264]
[INFO] [1662549440.441759]: param:/dev/ttyUSB0
[INFO] [1662549452.298103]: /listenerI heard hello world 1662549452.294159
[INFO] [1662549452.398594]: /listenerI heard hello world 1662549452.3944724
[INFO] [1662549452.498860]: /listenerI heard hello world 1662549452.4945526
[INFO] [1662549452.596540]: /listenerI heard hello world 1662549452.5943813

Please note that the original package in this post is taken from ros/ros_tutorials repository.

Please use the args attribute of the <node> tag. To assist you better, I made a sample code. Please see below:

I created my_args.launch file as shown below:

ravi@dell:~/ros_ws/src/ros_tutorials/rospy_tutorials/001_talker_listener$ cat my_args.launch 
<launch>
    <node pkg="rospy_tutorials" type="listener.py" name="listener" args="/dev/ttyUSB0" output="screen" />
</launch>

I edited the listener.py so that it can capture sys.argv[1] as shown below:

#!/usr/bin/env python
# license removed for brevity
import sys
import rospy
from std_msgs.msg import String

def callback(data):
    rospy.loginfo(rospy.get_caller_id() + 'I heard %s', data.data)

def listener(param):
    rospy.init_node('listener', anonymous=True)
    rospy.loginfo("param:" rospy.loginfo('param:' + param)
    rospy.Subscriber('chatter', String, callback)
    rospy.spin()

if __name__ == '__main__':
    listener(sys.argv[1])

Finally, I ran the launch file. I can see the value in the terminal. Please see below:

$ roslaunch rospy_tutorials my_args.launch 
... logging to /home/ravi/.ros/log/828983fa-2e9e-11ed-bbf8-d707e2da4a66/roslaunch-dell-90235.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://dell:33681/

SUMMARY
========

PARAMETERS
 * /rosdistro: noetic
 * /rosversion: 1.15.14

NODES
  /
    listener (rospy_tutorials/listener.py)

ROS_MASTER_URI=http://localhost:11311

process[listener-1]: started with pid [90264]
[INFO] [1662549440.441759]: param:/dev/ttyUSB0
[INFO] [1662549452.298103]: /listenerI heard hello world 1662549452.294159
[INFO] [1662549452.398594]: /listenerI heard hello world 1662549452.3944724
[INFO] [1662549452.498860]: /listenerI heard hello world 1662549452.4945526
[INFO] [1662549452.596540]: /listenerI heard hello world 1662549452.5943813

Please note that the original package in this post is taken from ros/ros_tutorials repository.