I received this question three times in past two days so time to answer it again. How does a photographer combine a flash exposure for subject in ‘shade’ and also set a correct exposure for the bright blue sky?
Rule 1) ALWAYS set the camera to Manual (M)!
Rule 2) With your ISO set to 100, preferably, set your aperture to F/22 and now with camera pointed at blue sky, adjust the shutter speed until a correct exposure is indicated. (chances are really good your shutter speed will be around 1/60 sec.), then take one shot with sky and subject but with NO FLASH...the sky will be a blue ‘perfect exposure’ and the subject(s) in front of the blue sky are now a silhouette. (image #2)
Rule 3) Okay so now with your flash OFF CAMERA (but ‘connected’ to the camera via a TTL coiled cord, or ‘connected’ via your own camera/flash wireless feature)set the Flash to TTL Mode. Because the Flash is still ‘connected’ to the camera, the Flash me KNOWS that you are at F/22 (and it knows what ISO you are at) and your FLASH only cares about what aperture/ISO you are using in so far as a correct flash exposure. All of the flashes built in algorithms are based on those two criteria, f/stop and ISO and so, at F/22 the Flash will put out just enough flash for your subjects based on F/22 so, while holding your flash in one hand, (which is hooked up to the camera via a coiled TTL Cord) and with your other hand holding the camera, fire away and just like that the same beautiful sky you shot a moment ago is just as beautiful the second time BUT instead of silhouetted subjects in the foreground, the subjects are now correctly exposed by the flash! Simple! You keep shooting!
Nikon D850, Nikkor 24-120mm, F/22@1/60, 100 ISO.