Thanks for contacting the Ad Manager API team.
What you may do is to check for the parameters of the method you're using. With this, you may check out our documentation for getAvailabilityForecastById where it's parameters can be seen. This should be the sample code for GetAvailabilityForecast (which is in Java) which you may check for reference. Furthermore, you may check out our available client libraries for reference here.
You can check our Forecasting guide for reference on the different types of forecasting that are supported in the Ad Manager API. If you would like to do an Availability or Delivery forecast, then the API supports forecasting on an existing or a prospective line item.
Regarding your requirement on getting the forecast for a specific dateRange, adUnitSize, adUnitId, & geography, if no existing line item contains the parameters that you need, then you will have to create a prospective line item instead. Again, you may check out the sample code for GetAvailabilityForecast (which is in Java) for reference and better clarity of the method via API. Furthermore, I would like to inform you that the Ad Manager API supports SOAP XML Request and Response Headers.
If you're still having trouble, I would request for you to kindly elaborate on the areas you're getting confused on so that we're able to address them accordingly. Also, if you're encountering discrepancies between the UI and API forecasting results, then please provide the following information below so I can further investigate via Reply privately to author option: