Available merge fields for email subject lines are salutation, first name and last name. All you need to do is to place double brackets around the name of the merge field.
For example your subject line might be: Thank you [[Salutation]] [[First Name]] [[Last Name]] for registering!
Any Sphere email which uses the editor can take advantage of these merge fields. That includes not only email blasts created under Communications > Email or Enewsletter but all advanced confirmation emails as well. please do not use special characters (like a comma: [[First Name]], ) because it will cause the field to be invalid.
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