Delphi custom format string

There is no built-in format specifier (or combination of them) that will do the formatting you're looking to do.

You can, of course, write your own function to do so (name it, of course, with something meaningful to the values you're formatting):

function MyFormat(Value: string): String;
begin
  Assert(Length(Value) >= 8);
  Result := System.Insert(Value, '-', 5);
  Result := System.Insert(Result,'-', 9);
end;

Use it:

Value := MyFormat('12345678');      // Returns '1234-567-8'
Value := MyFormat('123456789');     // Returns '1234-567-89'
Value := MyFormat('1234567890');    // Returns '1234-567-890'

If you insist on trying to do it with Format, you need multiple calls to Copy (although you can skip the first one by using a width specifier). These can be done, of course, on a single line; I've spread it across multiple just for formatting here to eliminate horizontal scrolling.

Str := '12345678';
Value := Format('%.4s-%s-%s', 
                [Str, 
                 Copy(Str, 5, 3), 
                 Copy(Str, 8, MaxInt)]);     // Return '1234-567-8'
Str := '1234567890';
Value := Format('%.4s-%s-%s', 
                [Str, 
                 Copy(Str, 5, 3), 
                 Copy(Str, 8, MaxInt)]);     // Return '1234-567-890'

There is no way to use a "width specifer" type method to extract substrings within a string, though. (You can extract the first n characters using %.ns, but you can't do the first n characters starting at the fourth with any combination of specifiers.)

Use Insert instead of Format:

Insert(s, '-', 5);
Insert(s, '-', 9);