Including Only Some Webform Answers in the Email Received

Hi,
I am trying to set up a custom template for the email received from the webform results. I don't want all of the webform results on the email and I have tried using this as an example to include the ones I want:

Firstname: %value[first_name]
Lastname: %value[last_name]

but that doesn't seem to work for me. In my form, the first field is "First Name:" I have tried using the code above with the space, and with underscores, but they come out the same. Is there something else I can try?

Thank you!