What would be the warrant for "To the Virgins, to Make Much of Time" poem?
A fascinating question. I assume you're using "warrant" in the sense of the Toulmin system of argument. If so, then the claim is in the title (make the best of your time). The warrant is the underlying value or assumption upon which the argument rests. In this poem, I'd say the warrant is "People want to make the best use of their time possible" or "People want to use their lives in the best way." The virgins to whom the poet speaks were trying to do so by being, well, virgins; he's trying to convince them to have fun instead, as a way of acting on the same warrant.