POST request to read data

The API I’m trying to access requires POST to read data, and I’m fairly confused on how to execute my calls. Also, the documentation re: how to access the service is slightly confusing too so my question touches on both parts.

Accessing the service
API documentation states to access the service by using the following:

<environment_url>/receipts/doit

The documentation continues to state every method call requires the following POST arguments: username, password, and method. There’s no reference re: tokens so I’m using Basic Auth in PM.

Now for actually making the call. I select the POST method and enter the prescribed url with the required method (getreceipts) and finally a parameter value which looks like this:

https://someurl/receipts/doit/getreceipts?reportId=010024294216

The request runs, forever, and finally returns with a 503 error stating “ftp.ftpexception: 503 login with user first”.

Looking for advice to (1) solve the 503 error and (2) understand if I need to be passing parameter data via the body since its a POST request to read data.