Adding anonymous properties

It is highly recommended to add identity, even though it is not required for adding properties, since this will let you identify and respond to recipients directly within our application. However, if you don't want identities, simply exclude identity, identity_type, first_name, and last_name

<input type="hidden" name="properties[color]" value="pink">
<input type="hidden" name="properties[device]" value="iPhone">