Parameters don't work in the API

I've created a project in which you can add reservations for conference rooms. I've set up an API in order to get all the reservations and to make reservations. In order to make this work I need to make use of parameters like StartDate and EndDate.

In the attachment I've added the request and the response in which I'm getting the following error: <faultcode>Client</faultcode> <faultstring>cvc-complex-type.2.4.a: Invalid content was found starting with element 'mak:StartDate'. One of '{StartDate}' is expected.</faultstring>

I've added the microflow and the response and output underneath.

I hope someone can tell me how to fix this error.