Obtaining the Access Token
In order to obtain one, you need to sign up for a Mapbox Account first: https://account.mapbox.com/auth/signup/
Next thing you need to do is to create a token
Name your Token, and leave out the Secret Scopes section. (also, no need to add an URL)
Once you press Create, you're gonna get redirected to a page where you can copy your access token.
Adding the Access Token to the site
The next step would be to go to your site, and on your Admin Dashboard, navigate to Settings > kvCORE > (kvCORE) Settings
And paste your access token to the Mapbox Access Token field. After that, hit Submit to save the changes.