i agree with the.real

You cant.
If you selected the characters from the "character map"
then the browser version doesnt support them.
If your a subscriber the best way to prevent this is to go to the browser, copy the characters, then press 
Ctrl + V in the text bar then send it, this will show either the characters you selected or boxes.
If the result was the characters then its ok to use, 
if not then try again.