For video guide, scroll down to the bottom of this post.
As of right now, at the time of writing this, you may not be able to see the map seed, like in the screenshot below:

However in such case there is still a way to directly extract the map seed from a save file. How I did this:

1. Find the location of your save game folder, mine using windows and the steam version is in Document/Humankind/Save Files.
2. The save file should be name.ctr, copy it.
3. Past the save file somewhere you can find it, such as to your desktop and replace ctr with zip, it should now say name.zip.
4. Open the name.zip file, it should look like in the image below (I'm swedish so the text in the image is swedish)
   
5. Take out the file called SaveDescriptor.bin and place it somewhere you can find it, such as your desktop.
6. Go to https://filext.com/file-extension/CTR and drop SaveDescriptor.bin to it like the instruction say. Scroll down and press on the button that say "show all bytes of this binary file". Once you have done that, scroll down again and press on the button that say "show all readable texts in this file.
7. Now it should look like the image below, I underlined the map seed with orange to make it easy to spot:
   
   
   Here is a video guide how to find the map seed