Do you mean when sump is low? Been long time since I programmed an Apex. I have Hydros but I do think the following will work:
Whatever you call the switch in this case SumpLo is switch name.
I suggest creating a virtual outlet with a Defer to filter out splashing etc temporarily causing the switch to become open for a second or two.
virtual outlet (ignore the [ ] and #, they are just comments in my source library for my benefit).
[SumpLow]
# VO to detect if the sump water level is low
#
Set OFF
If SumpLo OPEN Then ON
Defer 001:00 Then ON
You then add the following line to your return pump code to turn it off.
If Outlet SumpLow = ON Then OFF