Information on creating a Splashtop JSON can be found here under the installation files heading:
Navigate to your Profiles tab and click the plus sign under Available Payloads.
Select Android and then File
Name your payload, select Add/update file, specify the file path for your particular device type (\enterprise\usr\persist (For Zebra), \persist (For Non-Zebra), or \sdcard (alternate path for non-zebra without persist folder)), browse to your JSON under File, and select Save
Your file payload will now show under Available Payloads