"Please write it on the blackboard."
With 在, you put the preposition after the object it modifies:
在桌子上 - ON the table
在椅子下 - UNDER the chair
在我旁边 - NEXT TO me
在你后面 - BEHIND you
The question wasn't about it being a pre- or post-position (before or after the object); rather prepositional phrases in Chinese can come before the verb.
You are correct that 在 ordinary behaves like other coverbs (or "prepositions," if you prefer that term), with the entire coverbial phrase always appearing in front of the main verb. There are a handful of verbs for which 在 can occur after the verb. Ones that immediately come to mind are 住在 "to live at," 放在 "set at/on," and 生在 "be born at." This post-positional pattern also occurs with the coverb 给"give." (送给, 带给, etc.)
Did you by any chance write/pick 清 "clear" instead of 请 "request"? That tripped me up a few times...