Danger: a real brand never does it
"Add us as a manager of your channel": no, never
A brand or an agency asking for access to your YouTube channel, your password or your sign-in details "to schedule the video": the sign of a channel theft.
What it looks like
To schedule the video at the right time, please add us as a manager of your channel in YouTube Studio, or send us your sign-in details.
Why it is a sign
A manager can publish, delete, rename the channel and change who has access. With your sign-in details, it is even simpler. No campaign needs that: you are the one who publishes. YouTube's help says to be wary of any "marketing help" that asks for access to the channel.
What a real brand does
A real brand sends you the offer details, approves the video privately if agreed, and you publish. If it wants to see the video's figures, YouTube offers a limited brand partner access, which you grant from Studio and remove whenever you want.
What to do
Refuse, and never give your password, not even "temporarily". If access was already given, remove it in the channel permissions, change your password and check the signed-in devices.