Where's the mold function?  

Don't think it is possible in PS, never heard or seen something in that direction.
One idea is to make the mold in Cura and export the STL. Open up in PS and slice it there

GitHub

Posted by: @joshl

I'm trying to make a negative, like I could with cura. I can't find it in prusaslicer.

I looked through the features and did an extensive internet search and could not find this function.  I concur that is probably not possible.  You could request the feature on GitHub.  

https://github.com/prusa3d/PrusaSlicer/issues

There isn't a specific function for that.  The closest you can get is to add a basic shape such as a box or cylinder and then right click it, select Add Modifier> Load and choose the object you want to mould.  The set the perimeters, top, bottom and infill for the modifier object to zero.  That will cut out a negative of the modifier shape from the box/cylinder basic shape.
You could then use other modifier mesh shapes to add in fill channels etc.

Although it would be much better to actually create a proper mould object in some real cad software.