...if you must use the pop-up flash, use a white card to block direct light from flash to subject, angled so the light will bounce from the ceiling, which will act as a wide-angle diffuser. "White card" can be anything white, of a handy size. Once upon a time, I used a white napkin.
As others have already indicated, the larger, wider-bodied lenses will cast a shadow when using the pop-up flash. Some of my wide-angle lenses, with the hood in place, will manage to cast a shadow with a Speedlite/Speedlight mounted on the hot shoe. (I shoot with both Canon and Nikon, therefore the "Speedlite" part. ;-) )
If outdoors, and you must use the pop-up flash, for whatever reason, and the open sky is the background, invert your camera, taking the shot upside-down, as the shadow will disappear into infinity.
The best practice, of course, if flash is required for the conditions, is to get your flash off-camera, using a Speedlight, or other flash unit, with either a TTL cord, or using optical- or radio-triggered wireless. This will open creative possibilities that you have yet to imagine.
Congratulations on your new camera/lens combination!

I am a bit envious, as the D810, and Nikkor 14-24G, both remain on my wish list, rather than my gear list. I might manage to acquire one, or the other, but not likely both, this year.
And, most importantly, welcome to the forum!
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I wear a badge and pistol, and make evidentiary images at night, which incorporates elements of portrait, macro, still life, landscape, architecture, and PJ. I enjoy using both Canons and Nikons.