You'll probably need to unwrap the right side handlebar inboard of the right side brake-lever to install a trigger control.
I for one am not keen on using triggers or anything that resembles a brake lever for a throttle control on a kit built motorized bike. You'll also have to fabricate up someway to operate your clutch lever. These handlebars get cluttered very quickly if you start mounting lights, cyclometers and phone mounts, in addition to the two extra controls.
In panic situations there's the chance you might grab the throttle by mistake. It's easier to roll off the throttle with a twistgrip control, which is why fellow forum members here are recommending going with upright bars. Having a single lever to operate both brakes on the right is the standard practice also, Sunlight and Whatapart both sell these brake levers.
Both of my motorized bikes have twistgrips, but then I've been a motorcyclist most of my teens and adult life. Your chosen bike is not the best choice for a motorized bike, there's equally good MTB's and used upright bikes for sale that would be better for frame dimensions and quality of wheels and hubs. As a motorized road bike with an 80cc engine you're putting a substantial strain on parts designed to handle human power levels. The sidepull brakes this Schwinn is equipped with will have less effective braking than a set of V-Brakes or discs.
Brakes are essential on a motorized bike.
Edited to include a link to a new bike I just recently did an assembly and tune up on. It struck me as a half decent motorized bike project, It's almost twice as expensive but it has many features that make it superior to the Schwinn roadbike you listed. Plus it's upright bars and large 'V' type steel tubing frame that these CG engines require to mount securely. https://www.walmart.com/ip/Schwinn-...hguid=01679890-d95-16af1a26fa2efe&athena=true