This is due to the type of workflow. You have clicked on “Trigger workflow with”. See the documentation:
"For POST requests, this data should be set in the body of the POST request, unless you tick the ‘Querystring’ box, in which case the parameter is read as a query string from the URL.
GET requests do not have a body and the parameter must be read as a query string. Querystring is therefore automatically selected for all parameters when you trigger the workflow with GET. Enter a key and define the data type. If it is a list, tick “Is a list/array”. If this parameter is optional, tick “Optional”.
As @j.hennersdorf points out, your API workflows is configured for GET request (which don’t have a body, and therefore can’t receive parameters, aside from in the URL).