B737 flight crew reported departing with an incorrect squawk code due to fatigue.

Attributes
ACN	1649509
Time
Date	201905
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Night
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Takeoff
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 422
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

Narrative:

Holding number 1; tower asked us; '[call sign]; are you taking off VFR today?' we replied 'no.' we did not understand the reason for that unusual question; nor did we query the tower controller; nor did the controller elaborate. We were given a vector for takeoff and instructions to stay with tower frequency. After takeoff; tower (or departure control after switching; do not remember which); asked us to verify our squawk. We realize at that point we did not enter a squawk and what tower was covertly implying. We had not done a detailed departure briefing clearance. Fatigue was an issue.while the major fault was not setting a squawk and doing a detailed briefing; tower appeared they were aware of the error. They could have been handled it in a different or more elaborate manner. Also; ground control did not notice us taxiing without a squawk. The ramification was that there was an extra workload in retrieving the squawk and entering it during a higher workload and task saturated environment. Fatigue report submitted.not cutting corners in presenting briefings assuming procedures were done correctly. Understanding how insidious fatigue onset can be. Query controlling agencies as to unusual questions.

Original NASA ASRS Text

Title: B737 flight crew reported departing with an incorrect squawk code due to fatigue.

Narrative: Holding number 1; Tower asked us; '[Call sign]; are you taking off VFR today?' We replied 'No.' We did not understand the reason for that unusual question; nor did we query the Tower Controller; nor did the Controller elaborate. We were given a vector for takeoff and instructions to stay with Tower frequency. After takeoff; Tower (or Departure Control after switching; do not remember which); asked us to verify our squawk. We realize at that point we did not enter a squawk and what Tower was covertly implying. We had not done a detailed departure briefing clearance. Fatigue was an issue.While the major fault was not setting a squawk and doing a detailed briefing; Tower appeared they were aware of the error. They could have been handled it in a different or more elaborate manner. Also; Ground Control did not notice us taxiing without a squawk. The ramification was that there was an extra workload in retrieving the squawk and entering it during a higher workload and task saturated environment. Fatigue report submitted.Not cutting corners in presenting briefings assuming procedures were done correctly. Understanding how insidious fatigue onset can be. Query Controlling Agencies as to unusual questions.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.