To pick an icon other than your default, add "post-icon: keyword" as the first line of your reply.

So what if you're one of those "pretentious hipster" kids who has ★ᴄᴏᴏʟ ｆａｎｃｙ ϝσɳϯ i.e. stylized text for keywords that (usually) can't be manually typed out, especially not on a phone? Have your keywords in a handy cloud storage thing so it's possible to copy and paste them into the e-mail? Or is it time to redesign your keyword set?