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Tap the Settings app icon, which resembles a set of gears on a grey background. It's near the middle of the Settings page. You'll see the option at the bottom of the screen. Depending on the size of your iPhone's screen, you may first have to scroll down to see this option. This section is near the bottom of the page; it lists each address from which you can send an iMessage. At minimum, you will see one email address and one phone number here. The email address is your Apple ID email address. Tap the email address from which you want to send iMessages. This will ensure that, when you send an iMessage, the recipient of your iMessage will receive it from your email address and not your phone number. If the person on the other end of the message has your phone number in their contacts or can't receive iMessages, this method won't mask your number.
Open your iPhone's  Settings. Scroll down and tap  Messages. Tap Send & Receive. Review the "START NEW CONVERSATIONS FROM" section. Select an email address.