What are you actually trying to accomplish? Taking your question all by itself (without context) it seems pretty odd.
Brian.
He wants to 'tag' a playing file with a specific, pre determined tag, using a hotkey. I can't find anything in MCC that lets you do this but MCWS can
SetInfo
Set information about a file object.
Parameters:
File: The key of the file. (default: -1)
FileType: The type of value provided in 'File' (Key: file key; Filename: filename of file). (default: Key)
Field: The field to set. (default: )
Value: The value to set the field to. (default: )
List: Set to 'CSV' and comma delimit (RFC 4180) the field and value to set multiple values in one call. (default: )
Formatted: Set to 1 if you're passing a formatted value (like a formatted date). (default: 1)
I don't know if -1 is the currently playing file (it might be) so lets do this in steps. Ask JRiver for the currently playing file and get it's key, then run the SetInfo passing that file key + whatever field / value you're looking for. You can do this with autohotkey and curl probably. If you have problems with Curl popping up a prompt briefly, you can just use something like NirCMD to run it silently.