You'll need to activate that that option. To do that, you can modify your browser URL to look like this:
Note. To find your APPKEY, click on the following link for instructions on how to get it:
When you have it, replace APPKEY in the go.airship.com URL (above) with your own.
In the config screen, you have to toggle on the Custom Keys option within the configuration screen.
Once you have done this, you should then be able to see that field when composing messages :)
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