You'll have to refer to the Pre-Bass documentation for more details. But you could slide between the same C played on the E string and G below it. In this video, we compare and contrast the MM-Bass, Jay-Bass and Pre-Bass modules and amped versions of MM-Bass and Pre-Bass. So you couldn't slide between C played on the A string and G below it because that G doesn't exist on the A string. Also, just like a real bass, you can only slide between notes that exist on the same string. In the Scarbee products, the velocity of the first note you play (the starting note) in a slide sets the velocity for both notes and the velocity of the second note (the ending note) sets the duration of the slide. You have to set up automation or remote control to the plugin, and there are any number of ways to do that. Scarbee Pre-Bass and other Scarbee Bass instruments use CC #64 (Sustain) to trigger slides, which are actual samples of sliding between notes on a bass, but since FL doesn't natively send MIDI CC's directly to plugins, you can't do it directly from your sustain pedal or other MIDI controller knob or whatever.
FL has a native slide system where you can simply enter notes marked as 'slide notes' in the piano roll and the currently playing note will slide to it over the duration of their overlap.
