Basic Predistortion code for Saleh and Rapp nonlinearity

I want to find a way to easily predistort the input of a power amplifier in MATLAB. The amplifier is modeled as a Saleh or Rapp nonlinearity using the communications toolbox. I have tried the digital predistorter (https://www.mathworks.com/help/comm/ref/comm.dpd-system-object.html), but when I feed the result into my Rapp or Saleh amplifier, the output looks very nonlinear. I thought this would be an easy thing to do.

Later, I want to expand this approach to a real power amplifier designed in Cadence.