If you want to send an MP4 file as a text (SMS/MMS) or through a messaging app:
To provide the most accurate text for "18521.mp4," could you please clarify if you are referring to a specific video file you own or a viral video currently trending on social media? 18521mp4
If you are trying to add text overlays to a video file for a post, you can do so through the built-in editors of most apps: If you want to send an MP4 file
Are you looking to text from a video, or are you trying to add captions to a file you already have? How To Text People or Send Messages On TikTok 18521mp4
If "18521.mp4" is a video you want to share with someone via text/DM on TikTok: