You can try GPS spoof app (require root and DJI GO MOD) and set RTH to remote (if you want spark to have pre set RTH in case if you lost contact). But if you are sure that connection can be maintained, especially at RTH point, you can just use RTH to remote without gps spoof app.
Other way is GPS lock your final destination by physically beeing there and set RTH, and then later launch from start point.