How to get coordinates for a clicked location

I want to import mousePosition into Input.mousePosition and then convert the position of mousePosition into coordinates on the Cesium map What should I do??

First you need to turn that mouse position into a “ray” directed into the scene, and figure out what it hits. The “Raycasting” section on this page explains how to do both:

If you just need a Unity world position, you’re done. If you want to know the Earth-Centered, Earth-Fixed (ECEF) globe coordinates of that point, use the TransformUnityPositionToEarthCenteredEarthFixed method on the CesiumGeoreference component. If you want longitude/latitude/height, take the ECEF position and pass it to CesiumWgs84Ellipsoid.EarthCenteredEarthFixedToLongitudeLatitudeHeight. The returned X will be longitude, the Y latitude, and the Z the height above the WGS84 ellipsoid.