How to configure Conditional Mapping in AutoMapper?

AutoMapper allows adding conditions to properties that must be met before that property will be mapped.

Mapper.CreateMap<Source,Target>()
      .ForMember(t => t.Value, opt => 
            {
                opt.PreCondition(s => s.Value1.StartsWith("A"));
                opt.MapFrom(s => s.Value1);
            })

Try this

 Mapper.CreateMap<Source, Target>()
        .ForMember(dest => dest.Value, 
                   opt => opt.MapFrom
                   (src => src.Value1.StartsWith("A") ? src.Value1 : src.Value2));

Condition option is used to add conditions to properties that must be met before that property will be mapped and MapFrom option is used to perform custom source/destination member mappings.