How to call a process from a GET Web API ?

Hi All,

I am creating Document Download WebAPI which will be called by external system to download Appian Document . I want to capture few parameters such as data, time , document , user  whenever the document is downloaded (for an audit purpose) .I am trying to use a!startprocess or a!writetodatastoreentity() in the same webAPI but it doesn't work .

Can I call a smart service a!startProcess or a!writeToDataStoreEntity in web API's of the method GET?

What should be approach in this case ?

Any help would be appreciated. Thanks in advance!