FLT CREW OF EMB170 FAIL TO REPROGRAM RWY ASSIGNMENT PRIOR TO TKOF ON AKUNA RNAV DEP FROM DFW.

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MY FO AND I ARRIVED ONLY AT THE ACFT. WE DECIDED THAT HE WOULD BE PF. AS HE DID HIS WALKAROUND; I COMPLETED MY WORK AND ENTERED THE FLT PLAN IN THE FMS. PER OUR RELEASE; WE WERE ISSUED THE AKUNA 2 RNAV DEP. THE FMS REQUIRED THE INPUT OF A RWY IN ORDER TO INPUT THE REST OF THE DEP. I PUT RWY 17R IN THE FMS (AS A LOGICAL GUESS; BASED ON WHERE WE WERE PARKED). FO RETURNED. HE COMPLETED HIS WORK AND WE REVIEWED THE FLT PLAN. I STATED THAT I HAD INPUT RWY 17R FOR THE DEP RWY. NO SPECIFIC DEP RWY WAS ISSUED DURING CLRNC RECEIPT. ALL CHKLISTS AND PROCS WERE COMPLETED. WE PUSHED BACK AND CALLED FOR TAXI. GND CTL ASSIGNED RWY 18L. (THE DEP RWY WOULD HAVE BEEN MUCH MORE USEFUL TO KNOW WHILE WE WERE STILL AT THE GATE; RATHER THAN BEFORE OR DURING TAXI WHEN WE HAD A MUCH HIGHER WORKLOAD.) I WAS FOCUSED ON TAXI AND LISTENING TO ATC. AS WE APCHED RWY 18L; FO STARTED #2 ENG; WE RAN TAXI CHKLIST IN WHICH HE BRIEFED TKOF FOR RWY 18L. WE WERE CLRED INTO POS; THEN CLRED FOR TKOF. NEITHER OF US REMEMBERED WHAT THE CDI LOOKED LIKE IN POS ON THE RWY. FO WAS HESITANT ABOUT FOLLOWING FMS GUIDANCE (HE LATER STATED THAT 'SOMETHING' DIDN'T LOOK RIGHT) BUT STARTED A GRADUAL L TURN FOLLOWING FMS GUIDANCE. AS WE WERE HANDED OFF TO DEP CTL; CTLR WAS ALREADY SPEAKING; DEMANDING AN IMMEDIATE R TURN. WE CONTINUED TO FOLLOW ATC INSTRUCTIONS; AND IN THE MEANTIME REALIZED THAT WE HAD FAILED TO ENTER THE PROPER RWY FOR DEP.