How do I choose the current sourcing pin in CTMU? (pic microcontroller)
The CTMU function provides the current source internally to whichever analog channel is initialized.  You must initialize the ADC and analog channel before initializing the CTMU and enabling the current source, then perform the ADC conversion.  Look at the current calibration routines in https://ww1.microchip.com/downloads/en/DeviceDoc/Charge%20Time%20Measurement%20Unit%20CTMU%20DS-39724b.pdf for some example code.