Manu Online API Documentation
To create your own app, first sign up and create your application in this developer site (Account approval required). Get your Application ID and Application Key. Then go to your Manu Online account -> Admin -> Integrations -> Private apps and add your application id. (License Private apps if you have not already done so.) Finally get the account key from there and use this with the account id, application id and application key to authenticate your API call.
When you have these four pieces of information, you can try the API for your account. Enter the codes below, expand the endpoint you wish to try, enter any optional parameters and press "Try it out!"