We call these custom fields, and there are two ways to create them:
(1) Saving custom fields for one or a few contacts:
(a) Click the name of a contact in Relationships page
(b) Then, choose the down button next to "Unsubscribe" and then hit "Edit"
(c) Scroll down to the bottom of the edit page. You can then create a name and value for your field.
(d) Hit Save
(2) Saving custom fields for many contacts:
(a) Create a CSV spreadsheet with your custom fields in their own column.
(b) Follow the normal CSV upload process, stopping on our field mapping screen.
(c) Instead of one of our preset fields, choose Add a new Custom Field from the dropdown.
(d) Enter your desired field name and hit Ok
(e) Finish uploading your CSV. Your contacts will come through with the added custom field.
Now.... how to use your custom fields in a template:
Once your custom fields have been stored using the above methods, you'll be able to reference them in your templates.
(1) Create a template and click the Dynamic Fields button like you normally would.
(2) Select "Custom Fields" from the dropdown that appears
(3) You'll see your pre-stored custom fields and be able to select one to fill out in your template. Done!