Important Note: All emojis will render differently based the device and not all emojis are supported. Emojis currently work only in the subject line and not in the body.
To put an emoji in the subject line you need to use the hexademical code or hex code. Here is one example of a subject line using the emoji cake
Use a resource like Emojipedia (https://emojipedia.org/).
Find the codepoint of the emoji by scrolling close to the bottom page. The codepoint of cake is U+1F382
Replace U+ with &#x and add a semicolon or “;” at the end to turn it into hex code. For example, the cake emoji codepoint U+1F382 would turn into 🎂
Send a test email to see how the emoji renders in the inbox.