mc.AddProfile(new DtoToOtherProfile(new HttpContextAccessor()));

//OR

var mapperConfiguration = new MapperConfiguration(mc =>
        {
            IServiceProvider provider = services.BuildServiceProvider();
            mc.AddProfile(new DtoToOtherProfile(provider.GetService<IHttpContextAccessor>()));
        });

public DtoToOtherProfile(IHttpContextAccessor httpContextAccessor)
    {
     _httpContextAccessor = httpContextAccessor;

     CreateMap<MyDto, MyOther>()
    .ConstructUsing(myDto => new myOther(myDto.prop1, myDto .prop2,  _httpContextAccessor.HttpContext.User.Identity.Name)); // the name is what i intend to pass
      // other mapping
    }

public async Task<IActionResult> Create([FromBody]MyDto model)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        // save campaign
        var myOtherModel = _mapper.Map<MyOther>(model);
        // myOtherModel.UserName is always null
       // rest
      }

services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();

IServiceProvider provider = services.BuildServiceProvider();

var mapperConfiguration = new MapperConfiguration(mc =>
{
    mc.AddProfile(new DtoToOtherProfile(provider.GetRequiredService<IHttpContextAccessor>()));
});