There are two ways to create custom dynamic fields.
Saving Custom Fields for One or a Few Contacts
1. Click the name of a contact in the People page.
2. Choose the drop-down next to Unsubscribe and select Edit.
3. Scroll down to the bottom of the edit page. You can then create a name and value for your field.
4. Click Save.
Saving Custom Fields for Many Contacts
1. Create a CSV spreadsheet with your custom fields in their own column.
2. Follow the normal CSV upload process, stopping on the field mapping screen.
3. Instead of one of the preset fields, choose Add a new Custom Field from the drop-down.
4. Enter your desired field name and click OK.
5. Finish uploading your CSV. Your contacts will come through with the added custom field.
After you've created your custom field it can take around 30 minutes for the field to appear in the dynamic field drop-down in your template editor.
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.