Begin by completely signing out of all Google products and removing any account connected to YouTube within WellcomeMat's Connected Services.
Sign in to YouTube using the credentials that are exclusive to the account that you would like to connect. If it is a brand level account make sure that you have not signed in using a personal account.
Visit the Connected Services section of your account and add a YouTube account as normal. The correct YouTube account should now be connected. You can verify this by clicking the name of the connected account -- that will take you to the connected YouTube channel.
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