Text substitutions for a SendGrid template (SendGrid WordPress + Contact From 7)

My website has a quote request form built using Contact Form 7, and that dispatches an email via the WP SendGrid plugin. The templated email is dispatched successfully from SendGrid, but currently does not include the necessary form data.

My question is: How can I pass form data from CF7 to Sendgrid? In CF7s template, I tried the following line in the “Additional Headers” section:

substitutions:%first-name%=[first-name];%last-name%=[last-name];

Note: %first-name% and %last-name% exist as the placeholder text in the SendGrid template.

The above code does not work, but am I able to put text substitutions as a header like that? If this update needs to be made in PHP, should that live in the functions.php file? (I’m a front-end dev so I feel comfortable editing code, but have little WordPress/PHP experience).