It should be a GeoJSON file containing one or more polygons, in WGS84 Coordinate Reference System (this should be the default for most tools that produce such files).
With a bit of luck, after submitting your file you will find a corresponding MBTile file on the MBTiles page.
Use the square button on the top right corner of the map to draw a polygon. You can edit or delete the polygon by using the other top right buttons. Then you can continue to fill the form on the right
Please select a tile server:
Create a polygon GeoJSON file containing your Area of Interest (AOI). You can use QGIS or GeoJSON.io for this.