If you've used the preview version of Direct Line Speech or you're reading related documentation, you may expect to find a secret key on the Direct Line Speech channel registration page. The v1.7 DialogServiceConfig factory method FromBotSecret in the Speech SDK also expects this value.
The latest version of Direct Line Speech simplifies the process of contacting your bot from a device. On the channel registration page, the drop-down at the top associates your Direct Line Speech channel registration with a speech resource. Once associated, the v1.8 Speech SDK includes a BotFrameworkConfig::FromSubscription factory method that will configure a DialogServiceConnector to contact the bot you've associated with your subscription.
If you're still migrating your client application from v1.7 to v1.8, DialogServiceConfig::FromBotSecret may continue to work with a non-empty, non-null value for its channel secret parameter, e.g. the previous secret you used. It will simply be ignored when using a speech subscription associated with a newer channel registration. Please note that the value must be non-null and non-empty, as these are checked for on the device before the service-side association is relevant.
For a more detailed guide, please see the tutorial section that walks through channel registration.