Martin, you should have posted this as a new question.
Without knowing the version of DASYLab, the level (Full, Basic, Lite, Pro) or the device that you are communicating with, we can only suggest some general things.
Create a RS232 Output module and configure it for the correct communications port. If you just need to send a command, configure the module to send commands, not data. Then enter the command in the command box. Use \r to send a <CR> and \n to send a <LF>.
To use a data event or time event to trigger the RS232 module to send the command, you need to activate the setting to use an input. This input will look for 0 or 5, and 5 (TTL High) will send the command. You can also use an Action module to send the command without an input on the RS232 output module .
You can use a Trigger or a Comparator module to determine when to send the command, or, any other technique that will provide a TTL high for the RS232 module.