Now you can utilize those credentials in the Layer2 Cloud Connector with this connection string:
Url=MyUrl; Authentication=SPOnlineAppOnly; ClientId=MyClientId; TenantId=MyTenantId;
MyUrl is the url to the list or library you want to access.
You can find out your tenant id by logging into your tenant and then opening this link: https://login.microsoftonline.com/contoso.onmicrosoft.com/.well-known/openid-configuration
Again, replace "contoso" with the name of your tenant, then search for "token_endpoint" in the browser window. After that, you will find a URL that contains your tenant id: https://login.microsoftonline.com/YourTenantId/oauth2/token
Now paste your client secret into the Layer2 Cloud Connector "Password" field and you are done.