Hi, if you do not have a map select UI that automatically fills that parameter out for you, you are not on MZ 1.9.0. I would recommend to update to 1.9.0 unless you have a specific reason you can't use 1.9.0 (such as a plugin that is no longer being supported that does not work with 1.9.0).
However, you can still manually set this parameter if you cannot update to 1.9.0, the format is:
{"mapId":"1","x":"1","y":"1"}
Change the number for the map id, x, and y ONLY and do not touch anything else such as brackets or quotation marks or text as this is a JSON string and must be in exactly this format.
So for example if you want it to be map 2 at (3,6) you would put:
{"mapId":"2","x":"3","y":"6"}