To send it is necessary to add the text "[ussd]" (without quotes) in the message text. The answer comes from the same number to which you sent the USSD request.
A prerequisite is that USSD requests will only work with messages from the operator that display the final data, without additional questions from the mobile operator. In short, the message from the operator should have only one OK or Cancel button!
Also, to work requires Android version 8.0 and higher!