That nut doesn't just disappear, maybe you broke the top threaded area of the strut piston rod? The three outer bolts are how you remove the strut assembly.
Here's the Ford nut for $5.50 (not for Koni).
https://www.fordpartsgiant.com/parts/ford-hardware-miscellaneous_-n804608-s150.html
Here's a whole new
strut with a nut, also for $5.50.
https://www.rockauto.com/en/moreinfo.php?pk=6907&cc=1138968&jsn=549&jsn=549
Actually, expect to have to remove the lower control arm from the inner pivot, because the strut will be too long to get the knuckle out of the ball joint. Then to remove the strut with the knuckle attached (because you won't be able to get the knuckle off with the strut hanging in pieces), and then use a spring compressor to get the strut parts back together. Off comes the 30mm axle nut, tie rod end, sway bar end link, brake caliper, ball joint bolt, maybe strut rod bolt, etc....
If the bolt was deliberately taken off in error, you might be able to guide the threaded part of the strut back into the mount in-car, by slowly jacking up the knuckle and guiding the rod under the bellows back in place. You'll need to make sure the spring coil ends are oriented correctly in the strut and mount end as they go back together.
The nut is "tighten to 40-53 ft/lb"; it's tempting to go tighter. While holding the top of the rod with a small wrench to prevent the whole rod from turning, again to prevent breaking the rod.
I might have free nuts in about a week (off old KYBs) if my new-to-me Tokicos get the bugs worked out. Don't think it will matter, though, user last seen when he made this post.