Passing string into stdin of screen

I'm using 'screen' as a terminal emulator to communicate to a serial device.
However, I would like to script communication with this device.
Does anyone know how I can either:
1. start a screen terminal with a command line argument that will be
interpreted as stdin,
or
2. Pass a string into stdin of a running screen session?

My thought for option #2 was to use a combination of 'screen -X' and 'eval
STR', where STR is the string I want to pass in. However, none of the
following combinations were successful:
screen -X "eval STR"
screen -X "colon eval STR"
screen -X "colon"; screen -X eval STR"
screen -X "exec EXECUTABLE_SCRIPT_FILE"

Does anyone have any suggestions on how to correctly accomplish the above
task?
Any suggestions about how I can pass strings directly into the serial device
would also be welcomed.

Thank you,
William

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