To make a video post:
On the web: If it’s on Vimeo, YouTube, or some similar site you can just open a new video post, click the globe icon, and paste its embed code or video link into the post form. You can also click the camera icon to upload your own MOV or MP4 files—up to 100MB and 5 minutes per day.
On mobile: Open a new video post. Then pick a video from your camera roll, or record a new one.
PS: No judgment there but a friendly reminder that per the policies you agreed to when creating an account, we don't allow uploading sexually explicit videos of any kind to Tumblr.