Answer: Either, depending on the style you follow.
AP style is to omit the hyphen, but other sources like to hyphenate co, especially when it’s followed by a second o.
I prefer a hyphen in this one, myself, because coopt looks like an odd word and makes me think it should be pronounced like cooped. But since I’ve solemnly sworn to follow AP in my work writing, I guess I’ve got to do without.
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