Need to do fast blouse alteration

[Agatha60]

I've got a lovely M&S blouse which I plan to wear to a posh do in 2 nights time.
Just tried the blouse on again and the left side will just not stand straight and keeps flopping over! Aaargh!
Tried to starch it but it makes no difference.
Can anyone help with a fast fix please 🙏

Iron on interfacing or Wundaweb to stiffen the collar

Alternatively, pin/clip a chain across the opening to look like a necklace/chained brooch effect.