This is quite an old post, but I stumbled up on this looking for an answer; figured I put in what I have to date.
This is a great read:
https://community.home-assistant.io/t/howto-xiaomi-vacuum-zoned-cleaning/51293/31?u=mb_eqnvd3cjp
In Developer / Services, use the vacuum.send_command service to execute a an app_goto_target with coordinates as params.
Example:
entity_id: vacuum.roborock
command: app_goto_target
params: [32400,25500]
- Coordinates are relative to the dock. x=25500,y=25500 is the dock position. Moving the dock changes the game, so keep it steady unless you want to find these out again.
- Use the Mii App on your phone, and find the Pin & Go section.
- Launch a service call to 26500,25500 and see where the location marker is set on the map. You don’t actually have to wait for the bot to get there. Keep doing this until you have the target pin at the desired location.
Still trial an error, but at least not blindfolded 
Unfortunately I haven’t found anything better to date, e.g. can’t ask the bot (unrooted) to spit out actual coordinates.