The simplest route if you're not a graphic designer is to search 'She's Mine GIF' on Giphy or Tenor and download the first one that fits the mood. Honestly, most readers scrolling Wattpad on their phone just want that immediate visual punch of a possessive, angsty ship, they aren't analyzing the editing. Grab something with dark filters and intense eye contact from your fandom's show, slap 'SHE'S MINE' in a bold font over it using Canva's free video tool, and you're 90% there.
I see some creators making these incredibly intricate, multi-layered edits with custom lyrics and effects, and while they're stunning, they take hours. For a fic intro, the priority is setting the tone fast. A single, looping 3-second clip of a character looking protective or jealous, with the text timed to appear dramatically, does the job perfectly. Save the high-effort stuff for your cover art.