For POST requests, this data should be set in the body of the POST request, except if you check the 'Querystring' box, in which case the parameter will be read from the URL as a querystring. GET requests do not have a body, and the parameter must be read as a querystring. Therefore, querystring is automatically selected for all parameters when you trigger the workflow using GET. Enter a key and define the data type. If this is a list, check 'Is a list/array.' If this parameter is optional, check 'Optional.' When a request is received, the Bubble workflow engine checks the validity of the data and returns a 400 error if the data is invalid. Note that if the parameter is set to 'Querystring', it will not be made available for scheduling actions, this feature should only be used if the service that hits the API requires to add the parameter in the URL.
Access the value of the parameters in the subsequent actions of the workflows using the data source dropdown menu. You will see the parameters you defined at the top of the menu.