In general, when you want to assign controllers to interact with an Effect you’d assign that Effect as a Part Insertion Effect. And while Effect ON/OFF is not a programmable parameter, what normally is done with an Insertion Effect set to do Delays, is you can assign “Dry/Wet” Balance to a controller so that either the Effect flipped from Wet-to-Dry or Dry-to-Wet based on the position of the controller.
_ If assigned to a Switch (latch), you can toggle between two settings, or if you assign it to a Knob you can continuously vary the balance of Dry/Wet over a specific range.

If your Delay is assigned as a “shared” System Effect, where each Part has a Send amount to it (say you assign it as a Variation Effect) then you would simply manipulate the VAR SEND parameter for the Part, or Parts, in question, using a Knob assignment.
_ When you assign it to a Knob you can continuously vary the Send amount.

You will see “Dry/Wet” as an assignable parameter when the DELAY is assigned as an Insertion Effect.
You will see “Var Send” as an assignable parameter when the DELAY is assigned as a System Effect.

Assign Knobs in action
For an example of the DELAY setup as a “Var Send” (System) see the synth lead sound “Singleline 1”. Common AsgnKnob 1 will increase both Rev and Var Send. Light the [ASSIGN] button and turn Assign Knob 1 with upper Common selected.

For an example of the DELAY setup with “Dry/Wet” (Insertion) see the synth lead sound ”Wind Synth”. Part AsgnKnob 2 will increase Wet balance to each Ins A and Ins B. Light the [Assign] button with Part 1 selected.

You can change the “Dry/Wet” assignment to AsgnSw2 if you prefer a Switch (the AsgnSwitches are only available to be assigned on a per Part basis). With Part 1 *selected*, go to “Mod/Control” > “Control Assign” > with “Auto Select” active (green) turn Assign Knob 2 to recall the assignment. You can change the “Source” to AsgnSw2 for each Dry/Wet Destination.
Be sure to set the Assign Switch to “Latch” if you want the condition to hold... found in Edit Common > Part Settings > General

The Assign Switches will default to the Off condition (minimum) when a Performance is recalled. You can adjust the Ratio (positive or negative) if you would rather change stored condition — initially Wet or initially Dry.