There's API instructions and examples at this page. Scroll down to the section entitled marginBuy in the linked poloniex page, and you can find a way to place a margin buy order.
https://poloniex.com/support/api/marginBuy
Places a margin buy order in a given market. Required POST parameters are "currencyPair", "rate", and "amount". You may optionally specify a maximum lending rate using the "lendingRate" parameter. If successful, the method will return the order number and any trades immediately resulting from your order. Sample output:
{"success":1,"message":"Margin order placed.","orderNumber":"154407998","resultingTrades":{"BTC_DASH":[{"amount":"1.00000000","date":"2015-05-10 22:47:05","rate":"0.01383692","total":"0.01383692","tradeID":"1213556","type":"buy"}]}}
The stop/limit order instructions are in this section of that page.
buy
Places a limit buy order in a given market. Required POST parameters are "currencyPair", "rate", and "amount". If successful, the method will return the order number. Sample output:
{"orderNumber":31226040,"resultingTrades":[{"amount":"338.8732","date":"2014-10-18 23:03:21","rate":"0.00000173","total":"0.00058625","tradeID":"16164","type":"buy"}]}
You may optionally set "fillOrKill", "immediateOrCancel", "postOnly" to 1. A fill-or-kill order will either fill in its entirety or be completely aborted. An immediate-or-cancel order can be partially or completely filled, but any portion of the order that cannot be filled immediately will be canceled rather than left on the order book. A post-only order will only be placed if no portion of it fills immediately; this guarantees you will never pay the taker fee on any part of the order that fills.
