Hey stumpr,
I don't know what happened, but it works now. I just changed the vm_moref in the parameters to an actual instance of a vm, then changed it back. When I did that, I got an error msg saying that a parameter was wrong. So, I looked at the parameters and realized that I was using a Windows filepath instead of a Linux path. Changed that and viola, InitiateFileTransferToGuest worked! It returned a URL for me to send a PUT request to. Now, I just have to do the PUT request.
Thanks a bunch!