Go to the Google Cloud Console.

Create a new project (any name, no organization).

Go to Library and enable YouTube Data API v3.

Go to Credentials.

Configure consent screen: External. Hit Create.

App name: ytu.

User support email: Your email.

Developer contact: Your email.

Save and continue, twice.

Test users: Add users. Use your email and hit Add.

Save and continue.

Go to Credentials.

Create credentials: OAuth client ID.

Application type: Web application.

Name: use any name.

Authorised redirect URIs: Add URI, https://developers.google.com/oauthplayground

Create.

Don't close the page/tab.

Go to https://developers.google.com/oauthplayground

Hit the gear button on the right (OAuth 2.0 Configuration)

Force prompt: No.

Check Use your own OAuth credentials.

Copypaste Client ID and Client secret from the google console page above.

Close the configuration dialog.

On the left you will see Step 1 Select & authorize APIs.

Click YouTube Data API v3, select https://www.googleapis.com/auth/youtube and possibly https://www.googleapis.com/auth/youtube.upload

Hit Authorize APIs.

You will see the "Choose an account to continue to ytu" dialog. Select your youtube channel (NOT your gmail account). If you're doing this for your brand account, pick that instead.

You will see the "Google hasn’t verified this app" dialog. Hit Continue.

You will see the "ytu wants access to your Google Account" dialog. Hit Continue. If there are checkboxes, tick them.

Hit Exchange authorization code for tokens.

Grab your Refresh token and use it. "Step 2 Exchange authorization code for tokens" automatically collapses, you will need to expand it to see the token.