Assuming you mean analog AM transmitter, you use a float-to-complex, connect a constant source with value of 1 to IM input.
Then the audio source connects to RE input.
You will need an adjustable attenuator on the complex output before going to the sink device.
Using a multiply const with slider, adjust while monitoring the audio quality on AM receiver because it can easily overdrive.
You could also have issue with DC offset (IQ calibration) on sink. In that case you would need to use a multiply block to offset frequency on baseband.