To share from Combo to TikTok, you'll first need to authenticate your Combo account with your TikTok account. You can do this either from your Settings, or, when you finish editing a video.
To share a video to TikTok, you'll want to edit a video and press the share button when you're finished. Or, you can share an older video directly from your Inbox. Your video will be automatically sent to your TikTok account when it is finished rendering.
And that's pretty much it! In sum, you export your video from Combo, then when it's ready to be posted TikTok will send you a push notification.