TMDXRM46HDK: Does the HDK have a voltage or reference value for voltage in a register or other acces?

Hello,
No, it does not. You can use ADC module to implement such control from application. Hercules MCUs got Voltage monitor implemented on the device. When the voltage monitor detects a low voltage on the I/O supply, it will assert a power-on reset. When the voltage monitor detects an out-of-range voltage on the core supply, it asynchronously makes all output pins high impedance, and asserts a power-on reset.

Best regards,
Miro