You can use the Kentico.Xperience.Libraries NuGet package in your external application, and then call
CMS.Helpers.SecurityHelper.GetPBKDF2Hash, passing the password, which will return the hashed password.
The default iteration count is 10000. The salt is generated from the password itself running through
public Rfc2898DeriveBytes(string password, int saltSize, int iterations); in the