TestContract.cs

using Neo;
using Neo.SmartContract.Framework;
using Neo.SmartContract.Framework.Services;

namespace DevHawk
{
    public class TestContract : SmartContract
    {
        StorageMap domains = new StorageMap(Storage.CurrentContext, nameof(domains));

        UInt160 GetDomainOwner(string domain)
        {
            var value = domains.Get(domain);
            return (value == null) ? UInt160.Zero : (UInt160)value;
        }

        public bool Register(string domain, UInt160 owner)
        {
            var currentOwner = GetDomainOwner(domain);
            if (!currentOwner.IsZero)
            {
                Runtime.Log("Domain already registered");
                return false;
            }
            if (!Runtime.CheckWitness(owner))
            {
                Runtime.Log("CheckWitness Failed");
                return false;
            }

            domains.Put(domain, owner);
            return true;
        }
    }
}

TestContract.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Neo.SmartContract.Framework" Version="3.0.0-rc2" />
  </ItemGroup>

</Project>