FreshBooks connects to a wide array of tools and services. You can find the list from http://community.freshbooks.com/addons/
Each add-on has different specific requirements, but they all require two pieces of information:
See below for how to get these pieces of information.
If you are asked for your account URL, it is the same thing as your login page URL. Depending on your setup, it will look either like https://joesgarage.freshbooks.com or
https://joesgarage.billingarm.com. If you don’t know what your login page URL is, you can search for it with your email address.
Your API URL is the location on the Web where the add-on talks to your FreshBooks account. You probably don’t need to know anything about it except where to find it so you can copy and paste it into the add-on. Instructions to retrieve it are below.
Your Authentication Token is a verification code that allows third-party software and services to update and retrieve your account data. Your Authentication Token should be kept secret and only entered into software and services that you trust. Instructions to retrieve it are below.

Your Authentication Token is linked to your account password. Just change or reset your account password and your API key will also change. You will have to provide the new Authentication Token to all the add-ons you continue to use.
If you are an administrator, you can also disable all add-ons by following the same steps above but choosing to uncheck the checkbox next to “Yes, I agree to the API terms of service”.