How do I "formally accept" a job offer?

Hi all. Just got the official offer from the job I really wanted! Yay! Salary negotiation and such has already been dealt with and I am satisfied with the offer.

In the offer email, something along the lines of "Should you formally accept this offer, I will send you more information..." etc etc. Emphasis is mine.

What does formally accept mean in this context? I was not referred to any online portals and no specific instructions were given. Some resources online have templates for emails to accept offers, but is that considered formal?

Just thought I would ask here (since its now past 5pm on the day before a holiday anyway) to see if anyone else had some advice.

Thanks!

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