Google Tag Manager
MakePlans can automatically send visitor data to your Google Tag Manager account. All you need is to enter you GTM ID. To find your container ID see https://support.google.com/tagmanager/answer/6103696?hl=en. Note that you only need to enter your container ID - not the code snippet (JavaScript).
Under 'Setup -> Booking site' you will find an input for entering your GTM ID.
Track conversions with GTM
When a booking is completed MakePlans will trigger the following event: 'booking_completed'.