To offer online booking through your Google "My Business" profile, follow these steps:
In your business profile section, scroll down to the "Posts" section.
Click "Add a post"
Click "Add a button", making sure to select "Book" as the button option.
Enter your booxi site link in the "Link for your button" section. You can find your booxi site link in your booxi account: "My Business" tab > Business Details.
When ready, click "Post".