how can i add a subscript string to a textform in flutter?

As I can see you have used a few in the subscript letters. I would suggest creating a map of those and use it in your code. As we can’t have something for ₘᵢₙ we can use Unicodes in combinations.

  Map<String, String> ksub_map = {
    'min': '\u2098\u1D62\u2099',
    't': '\u209C',
    'e': '\u2091',
  };

You can retrieve it like this in your code.

  print(ksub_map['min']);  // ₘᵢₙ
  print(ksub_map['t']);  // ₜ
  print(ksub_map['e']);  // ₑ