When you purchase the payment plugin, you will receive a zip file called os_eb_squarehostedcheckout.zip. Go to Events Booking -> Payment Plugins to install it as any other payment plugin. See https://docs.joomdonation.com/eventsbooking/basic-setup/payment-plugins#install-new-payment-plugin for installation instructions if needed.
Go to Events Booking -> Payment Plugins, click on os_squarehostedcheckout payment plugin to edit and enter the following parameters:
Signature Key: You can get data Signature Key data of the webhook which you create in next step.
Order Prefix: This is optional. You might want to enter a prefix for this parameter to make sure payment note passed to Square unique.
Inside the app, from left menu, go to Webhooks >> Subscriptions, and add a new webhook subscription
(Of course, need to replace https://domain.com/ with your site url)
After creating webhook, get Signature Key of the webhook and enter it into Signature Key parameter of the payment plugin.