Probably the same as Morph2. From the Morph2 manual..
"1) Create two audio tracks to play back the audio files you want to morph between, and
place your audio files on them
2) Set track #2 to “no output”
3) Insert MORPH 2 on track #1 by dragging it from the device browser to the track
4) Select track #2 as sidechain input by first clicking on the “assign side chain” button!
5) Then select Track # 2 as sidechain source"
The manual has screenshots for each step, so if you can't figure it out download the manual. If you can't get that, send me a PM with your email address and I'll send you the Morph2 manual.