const TypeAttributes typeAttributes = TypeAttributes.Public |
                                      TypeAttributes.SequentialLayout |
                                      TypeAttributes.UnicodeClass;
var typeBuilder = moduleBuilder.DefineType("MyType", typeAttributes, typeof(ValueType));

ConstructorInfo structLayoutAttributeConstructorInfo = typeof(StructLayoutAttribute).GetConstructor(new[] { typeof(LayoutKind) });

FieldInfo charSetFieldInfo = typeof(StructLayoutAttribute).GetField(nameof(StructLayoutAttribute.CharSet));
CustomAttributeBuilder attr = new CustomAttributeBuilder(structLayoutAttributeConstructorInfo,
                                                         new object[] { LayoutKind.Sequential },
                                                         new FieldInfo[] { charSetFieldInfo },
                                                         new object[] { CharSet.Unicode });
typeBuilder.SetCustomAttribute(structLayoutAttributeBuilder);

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]