Profile settings → Social accounts. Each network has a card with a Connect button — it opens the network's own authorization page; you sign in there and the account appears in the card, ready to publish to.
The Instagram rule (Meta requires it)
Instagram publishing only works for Business or Creator accounts linked to a Facebook Page. A personal Instagram can't be connected by any tool — that's Meta's rule, not ours. Switching is free and takes a minute: Instagram → Settings → Account type. On the Facebook picker, tick only the Instagram account that belongs to this brand profile.
Your client's account — without their password
Managing someone else's account? Under each network's Connect button there's “Client's account? Invite them”. Enter their email — they get a branded invitation explaining everything, click it on their own device, authorize on the network's page, and the account lands in your profile. The link works for 7 days and can only attach an account — it grants no other access. When they finish, you both get a confirmation email.
Prefer a messenger? Copy link gives you the same invitation as a URL.