using System;

using AutoMapper;

public class Program

{

    public static void Main()

    {

        var config = new MapperConfiguration(cfg => cfg.CreateMap<Source, Target>().ConstructUsing(s => new Target(s.Prop)));

        var mapper = config.CreateMapper();

        var source = new Source { Prop = "VALUE" };

        var target = mapper.Map<Target>(source);

        Console.WriteLine(target.Prop); // should be TEST_VALUE, but is VALUE because PREFIX == null in the ctor

    }

    public class Target

    {

        private static readonly string PREFIX = "TEST_";

        // same issue with:

        //private static readonly string PREFIX

        //static Target()

        //{

        //  PREFIX = "TEST_";

        //}

        public Target(string prop)

        {

            this.Prop = PREFIX + prop;

        }

        public string Prop { get; private set; }

    }

    public class Source

    {

        public string Prop { get; set; }

    }

}