Add Online Booking to Google My Business

Log in to your Google My Business listing: https://business.google.com

If you don't have an account, you can create one for free here: https://business.google.com

Fill in your business name and and contact info. Note: in order to see your changes live, you will be asked to verify by having a code mailed to your address. While you're waitinfg on verification, you can fill in the rest of your profile:
In the left menu, click "Info"
Scroll down to "Appointment URL" then click the pencil icon to edit your URLs
Under "Appointment URL" paste in your Bookedin link. It will look something like this: https://bookedin.com/book/your-business-name-here (copy this from your Bookedin account settings. Need an account? Click here)
Click "Apply" and that's it! Once you are verified, your clients will be able to book appointments directly from Google Maps and your Google business listing.