Here's how to easily add any custom field to Booker25's Reservation form
In order keep configuration simple, please perform the following steps in Salesforce Classic (Booker25 is Lightning-Ready, you may afterwards use the app in Salesforce Lightning as you please)
Step-by-step guide
Go to Setup > Create > Objects
Click on Reservation
We will assume the field you want to add to the Reservation form already exists, or that you know how to create custom fields in Salesforce
Scroll down to the Field Sets section
Click edit next to the Custom Fields field set
Drag and drop the custom field you want to add onto the field set layout
Click Save
Your custom field will now show up in the Custom Fields section on the Reservation form on the calendar.