You can directly send the states "NO" and "NC" to the ThingSpeak channel field as they are. ThingSpeak fields accept non-numeric entries too, however you may see that the field visualization does not update. To see the field value you can send a GET request to get the field value as shown here: Get a Channel Field Feed.
Alternatively, you can encode these two states as either 0 and 1 and send the numeric value to the ThingSpeak field, so that now you can see how the field visualization updates between these two states.
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