1.
by all means, without fail, at any cost
An emphatic form of ぜひ, expressing a strong desire or request. Conveys that the speaker is very earnest about wanting something to happen.
Usage
是非とも is a stronger, more emphatic version of 是非 ("by all means, definitely"). It adds extra urgency or earnestness to requests and wishes. Often used in formal or polite contexts to show sincerity.
COMPARISON WITH 是非:
Register
Slightly more formal than 是非 alone. Common in business settings and formal invitations.