It took me a while to dig this up; SO I'm sharing.

The problem with copying and pasteing emoji's into the subject line is the risk of hitting spam traps. To avoid them, you can insert the subject as a MIME attachement via https://www.base64encode.org/

Example
Desired Subject Line: Extra 25% Off Outlet ★ Ends Tomorrow ★
Encoding Result: RXh0cmEgMjUlIE9mZiBPdXRsZXQg4piFIEVuZHMgVG9tb3Jyb3cg4piF
Complete Link to paste in Subject Line: =?utf-8?B?RXh0cmEgMjUlIE9mZiBPdXRsZXQg4piFIEVuZHMgVG9tb3Jyb3cg4piF?=