Get Random Position on NavMesh in Unity

NavMeshUtil.cs

using UnityEngine.AI;
using UnityEngine;

public static class NavMeshUtil {
    
    // Get Random Point on a Navmesh surface
    public static Vector3 GetRandomPoint(Vector3 center, float maxDistance) {
        // Get Random Point inside Sphere which position is center, radius is maxDistance
        Vector3 randomPos = Random.insideUnitSphere * maxDistance + center;

        NavMeshHit hit; // NavMesh Sampling Info Container

        // from randomPos find a nearest point on NavMesh surface in range of maxDistance
        NavMesh.SamplePosition(randomPos, out hit, maxDistance, NavMesh.AllAreas);

        return hit.position;
    }
}

If you don't mind having the "random" points in a grid, you can pregenerate points according to the navmesh's bounds and generate points using Mathf.PerlinNoise

Afterwards you can exclude all points that aren't found by NavMesh.SamplePosition and then choose a random valid point with Random.Range(0, pointList.Count)