This article highlights how to use first name, last name or any other custom fields in your email.
The personalisation tag format is
[first_name,fallback=user]
The above can be used in both - subject and body of the email.
Example
Hey [first_name,fallback=user]
You subscribed to us using the email [primary_email,fallback=]. Hence we are reaching out.
Adding a fallback using Drag and Drop UI
Step 1: Select the text and click “Merge Tags”
Step 2: Select the custom field