Sending unsolicited whispers is highly likely to be detected as spam and the messages get blocked. As this would be your own Twitch account you may end up blocking your ability to send whispers yourself too if you get flagged as a spam account.
I better solution would be to do this through an external website where there user goes through the OAuth flow, and your site checks if they are a sub or not and then provides them the code for a reward/gift that way.
Depending on usage it may also be possible to do this within an extension too, you would need to read the extension guidelines to make sure it would all be compliant.