What DKIM is
DKIM adds a digital signature to every email, published as a public key in DNS at selector._domainkey.yourdomain. Inboxes use it to confirm the message really came from you.
Selectors
Each provider uses its own selector name (e.g. google, k1, s1, selector1). Enter the one your email provider uses.
Honest note: DKIM selectors vary by provider — a “not found” result may just mean you used the wrong selector, not that DKIM is missing.