You can't do exactly what you want, but you can get pretty close by specifying mapping options when you call Map. Ignore the property in your config:
cfg.CreateMap<Message, MessageDto>()
.ForMember(dest => dest.Timestamp, opt => opt.Ignore());
Then pass in options when you call your map:
int someValue = 5;
var dto = Mapper.Map<Message, MessageDto>(message, opt =>
opt.AfterMap((src, dest) => dest.TimeStamp = src.SendTime.AddMinutes(someValue)));
Note that you need to use the Mapper.Map<TSrc, TDest>
overload to use this syntax.
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