Takes about 60 seconds. You'll need the Google account that manages your business.
On your phone or computer, make sure you're signed in to the Google account that manages your business, then search for your business name. Your profile panel appears with “You manage this Business Profile” at the top.
In the row of buttons on your profile panel, find and tap “Ask for reviews”. On some accounts it's called “Get more reviews” — same thing.
Google shows your short review link — it looks like g.page/r/…/review. Tap Copy, come back, and paste it into the signup form. That's it.