meanwhile i got the following answer from their support. Didn't try it out yet, because i am focussing on other exchanges now. If you tried it or have found another solution, i would be thankful for a comment.
The v1.1 API currently does not have the ability to place time in effect orders. There is an unsupported 2.0 API that has this feature. Please be aware that this was not intended for public use at the time it was built. This means you will be using these endpoints at your own risk.
Below is an example buy with a time in effect IOC order. I have also included a call to the sell endpoint.
OrderType
LIMIT
TimeInEffect
GOOD_TIL_CANCELLED,
IMMEDIATE_OR_CANCEL,
FILL_OR_KILL
https://bittrex.com/api/v2.0/key/market/tradesell?marketname=BTC-LTC&ordertype=XXXX&quantity=1&rate=.00000012&timeInEffect=IOC&conditiontype=xxx&target=xxxx
https://bittrex.com/api/v2.0/key/market/TradeBuy?marketName=BTC-LTC&orderType=LIMIT&quantity=5000&rate=.00000012&timeInEffect=GOOD_TIL_CANCELLED&conditionType=NONE&target=0
Best Regards,
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