Back test how to write Entry script always as Day Open Price in 15 min chart

Please help me on tradescript
Chart 15mins
BUY script = Day Open price (always)

How to get day open price in 15mins chart

the above condition included with two time frames like data open and with 15 minute chart ,
tradescript works with OHLC values and it is not possible to define conditions with multiple time frame in single script