Since the dawn of time humans have felt the urge to add effects to their tracks, and frankly, music production wouldn't exist without them. Soundtrap includes a wide range of the effects you'd find in any professional recording studio, and they can be added to any track.
First, make sure you're in the Instrument view (1). You open it by selecting a track and clicking either on "Show instrument" or on the instrument icon in the track header (2).
Click the purple "Effects" button (3, 4) to open up the effects dialog. Here you'll see all the effects for the track. Click "Add effect" in the empty effect slot (5) and choose the desired effect by clicking on it in the Add effect menu (6). (If you're already using more than three effects, you will need to navigate with the little right arrow in order to find the empty effect slot.)
(1) Instrument view
(2) Show instrument, instrument icon on track header. Click either to open Instrument view.
(3) Effects button (Voice & Microphones track)
(4) Effects button (Instrument track)
(5) Effects dialog (click "Add effect" to add a new effect. If you can't see it, use the arrows in the dialog to get to the rightmost position.)
(6) Add effect (click an effect to add it)