All you have to do this copy this into something like Microsoft Visual Studio Code,
change the arguments that need to be changed, then save the file as a .html file.

Run this file in your browser. By clicking send it should send your message to your webhook.
Make sure you have changed the following arguments to the things set below:
Change the link after request.open("POST" to your Discord Webhook URL.
Change the text after username: to the name you want your webhook to have in chat.
Change the test after avatar_url: to a direct image link you want your avatar to have.
Change the text after content: to the message you want your webhook to send.