How To Get Text First Frame Of Video

im posting on tiktok now, i dont use the mobile app. im having trouble finding an easy way to put text at the beginning of a video so it shows like a thumb when you post vids

sites online

or i use mac, and imovie i figured it out once which title theme did that but i keep trying and cant get it to do it again

Hope this link will help:
https://support.apple.com/guide/imovie-iphone/add-titles-kna14aaa4db/ios

Try ffmpeg. Plenty of resources on the Internet and available for macOS too.
https://ffmpeg.org/

id like to just use a free wesite