What's the ASCII character code for '—'?

Quotation from wiki (Em dash)

When an actual em dash is unavailable—as in the ASCII character set—a double ("--") or triple hyphen-minus ("---") is used. In Unicode, the em dash is U+2014 (decimal 8212).

Em dash character is not a part of ASCII character set.

— is known as an Em Dash. It's character code is \u2014. It is not an ASCII character, so you cannot decode it with the ASCII character set because it is not in the ASCII character table. You would probably want to use UTF8 instead.

This character does not exist in ASCII, but only in Unicode, usually encoded by UTF-8.

In UTF-8, characters are encoded by 2- or 3-byte sequences (or occasionally longer), where none of the two or three bytes is a valid ASCII code, where all of them are outside the ASCII range of 0 through 127.

One suspects that the foregoing only partly answers your question, but if so then this is probably because your question is, inadvertently, only partly asked. For further details, you can extend your question with more specifics.