[EventSub WS] 400 Bad Request - The value specified in the method field is not valid

API
2

Most of your Subscription request is in a header isntead of the body
And you have a slew of extra JSON stringify’s

Should be more like:

function subscribeToEventSub(event_name: string, version: string, condition: any = {}, app_token: string, session_id: string): void {
    fetch('https://api.twitch.tv/helix/eventsub/subscriptions', {
        method: 'POST',
        headers: {
            "Content-Type": "application/json",
            "Authorization": "Bearer " + app_token,
            "Client-Id": "<APP CLIENT ID>"
       },
       body: JSON.stringify({
            "type": event_name,
            "version": version,
            "condition": condition, // {"broadcaster_user_id": user_id}
            "transport": {
                "method": "websocket",
                "session_id": session_id
            }
        }
    })

Also note that version in the function call should be typecast to a string so you can handle version beta and Twitch hasn’t said they’ll always be numeric in style.

You can refer to my code example here which might help you out:

https://barrycarlyon.github.io/twitch_misc/eventsub/websockets/web/basic/

Yeah as you noted, you are using the wrong token type.

WebSockets only accepts a user token.

Sounds like you generated an implict auth token with a different clientID to the one you tried to use in code