If you have received a discount code from from another business, you will be eligible for a discount on your first subscription invoice.
There are a two ways to use a discount code. Assuming your code is SAMPLECODE, you can do so either by:
copy/pasting the url they gave you into your browser or simply click on it before signing up. Your discount code URL would look something like this: https://bookem.com/oc=SAMPLECODE.
This will automatically add your discount code and you'll see it show when you click Sign up:
Or you could use only the code and manually enter it when signing up. Click Sign up while visiting bookem.com; then:
Click on 'Have a discount code?'
type in your discount code and hit 'Apply'
Note, discount codes are only usable once for a business, and only when signing up.