Second syllable, and I’ve never heard it any other way. I grew up in New England, went to school in the Midwest, live in California, and have attended many conferences attended by people from all over the country (at which exhortations to contribute to the group were common), and I have never heard a single soul use “CON-tri-bute” as a verb. We are speaking about a verb, right?
As with many other nouns and verbs formed from the same roots, the accent shifts when we go to the noun form: CON-tri-BU-tion (third syllable). If you dropped the noun ending, you’d have CON-tri-BUTE, probably accenting the first. But we don’t build words this way.
However (thanks in part to technology, I fear), we are seeing more and more use of verbs in place of nouns, even among educated people. These grate on my nerves, so I am very aware of them. Most common outside of technical jargon, I think, is “invite”:
Let’s in-VITE Joe and Maggie. (verb)
Did you get the IN-vite? (used as a noun in place of “invitation”)
English-speakers seem instinctively to shift the accent, as @gasman points out. There are many other examples: perfume, rebel, detail, permit, address, and so on, not to mention invalid, which is a slightly different case. We shift the accent when we add syllables (invoking old Latin rules, I believe—placing the accent in relation to the end of the word) as well as when we change meanings.