Thanks to our body and its physiological reflexes, which will not let us just keep our finger in boiling water until it is completely "welded". A person will make every effort to stick him out of there, and, thereby, save his limb.
If, for some reason (think for yourself, I know you can) he cannot do this, a person, most likely, as with many inhuman tortures, after several terrible minutes, during which he will endure, or actively express his emotions about this, he will simply fall into anaphylactic shock from the unheard-of abundance of painful feelings and sensations.
Not a finger, but a hand, not welded, but fried before, they say, charring Guy Mutsiy Scsevola. Than saved besieged Rome in 509 BC. - Shocked by what he saw, the Etruscan king of Porsena withdrew the troops from the city. The young man, by the way, survived, otherwise why would he be called afterwards "Scovola" - "left-handed". More details can be found in Titus Livy.
A more radical experiment has been recorded in Russian literature - Ivan the Fool sighed both in boiling water and boiled milk, which Pyotr Ershov described in detail in The Little Humpbacked Horse. But that is already a fairy tale, you know ...
It will hurt.
And if it's not your finger yet, there will be a court for bodily harm that caused the victim's incapacity for work, as well as for the use of torture and experiments on people.
In general, nothing good will happen.