When using a custom coded email, if you want to add a personalization token, you'll need to manually enter this into the body of your email. When manually typing in your token, keep in mind that the token needs to contain the supported variable along with the contact property name you wish to use for personalization.
For example, if you want to use the First Name contact property, you'll want to accompany the name of the property with its appropriate variable. Your token will look like this within your custom email:
If you want to set default values for any personalization tokens being used in a custom code email, you can do this in your email settings. You can learn more about how to do this here.