how to upload image? (lemmy fetch api)

yup. me again...
\ client
\ proxy server (for cors)

i cant install lemmy-js-client because i have js. not node.js. i already tried some CDN and ESM libraries but it needs exports from node.js. i also tried to create some sort of lemmy-js-proxy (server with lemmy-js-client) which i would fetch through my site with the js but... JS CANT FETCH IT BECAUSE 404! i dont know why...