Thank you for the question.
You can set your command using the alarm button. Unfortunately, there will be some minimal information appended
I configured the device to send "testest (device imei)" by altering two options:
2. Keyboard->Alarm Button
Make sure to enter your number entered in SMS \ Call Settings -> GSM predefined numbers list
Enable SMS Data Sending, Select your number in Features->Alarm
Possible command configuration options can be accessed by hovering SMS Text window in Features->Alarm section of the configurator and pressing F1 on the keyboard. I also attach this below:
Additional information: https://wiki.teltonika.lt/view/TMT250_Features_settings
If you have any further questions don't hesitate to ask. Alternatively, upvote the answer if it provided a solution to your question.
Stay awesome and have a great day. :)