When sending emails, many senders wish to add a personal touch by dynamically inserting the recipient name into the subject line or greeting.
In order to insert a recipient's First Name in your emails, use the Tags section of the Single Send to grab the appropriate tagging. When you copy the Tag for First Name, it initially includes a default placeholder (which is optional.)
Using the format will automatically reformat to this format in the Single Send UI:
{{ insert first_name 'default=default' }}.
Below are some examples of handlebar tags that will allow this replacement function, and you can learn more about this in detail in these documents here:
Single Sends - Substitution Tags with the Design Editor

Clicking the tag will auto copy the required tag or you can copy this:
{{ insert first_name 'default=default' }}
Pasting this in the Subject line, this would render like the following:
When keeping the default-placeholder Tag as-is:
Hello {{ insert first_name 'default=default' }}!
A person whose contact field contains the first name "Sam" would render this way:
Hello Sam!
A person with no first name would render this way:
Hello default!
If you use the default placeholder tag, you must add a default value as well.
Modifying the default in the Tag:
Hello {{ insert first_name 'default=you' }}!
A person whose contact field contains the first name "Sam" would render this way:
Hello Sam!
A person with no first name in this case would render this way:
Hello you!
You can also use a simple handlebar tag of {{first_name}} (deleting insert and 'default=default' and any extra spaces from the Tag) to dynamically insert the recipient's name.
Notice: there's no need to use 'insert' if you're not using the default placeholder.
If the recipient does not have a first name listed in the contact, where the Tag is, it will render blank.