Intercom requires both enablement and configuration in Valimail's platform as well as follow up steps the account owner must perform in the Intercom application.
In order to enable Intercom in Valimail, navigate to your domain's configuration page. Under Enabled Senders, click the + (plus sign) and add Intercom as a sender if you have not already.
Towards the bottom of the page click Add a DKIM key and add the DKIM key(s) provided from the Intercom platform. If you have not created the keys or do not have the values of the keys, log in to the Intercom application to get this. It is usually in the form of a CNAME and will likely be called "intercom". Be sure to associate the key with the Intercom service when adding it to Valimail.
Once the key has been added to the platform you will need to verify the setup in the Intercom application. For more information please see Intercom's instructions here. Please note, there may be a 2nd CNAME record for you to add to your own DNS in order to Verify your domain per that link.