Apparently YouTube blocked the IPs of the several Lemmy servers such as Lemmy.World, so you cannot just conveniently post a video and get title and thumbnail filled out. Basically, these tasks need to be done locally instead of the Lemmy server. If you use a Lemmy client such as Thunder and use the share sheet from YouTube with it, the title is already filled out.

Here are two bookmarklets you can use. The way bookmarklets work is that you create a new web browser bookmark and paste the JavaScript code where the URL would go. Something like this:

The first bookmarklet is a generic one and should work for everyone regardless of browser choice. When in the YouTube browser tab, you just click on it and a new tab opens to create a new post. Title and thumbnail are filled out.

You need to replace "lemmy.world" with the domain name of your Lemmy instance.

I cannot post the code directly in code markup because stupid Cloudflare thinks I'm doing an SQL Injection. 🙄

Code: https://pastefy.app/SgP7s3Cp

The second bookmarklet is for when you use Firefox Multi-Account Containers, for example when you're logged into YouTube using a dedicated account and you don't want it to affect tabs with Google services you might use with a different account. If you don't have that requirement, just use the first script.

Code: https://pastefy.app/TKEbB7Jx

The way Lemmy works, you may be prompted to login again. Lemmy can do weird things like ignore your session when referrer isn't sent. Still easier than filling out the video title yourself, IMO.

Code is public domain.