This procedure should be very safe. This should not break anything, or delete your installs (Linux or Windows).

Checking Partitions

Here, we want to check out our partitions, and identify a couple of things.

We're particularly looking for two partitions. We want to find the EFI partition for Windows; that is nvme0n1p1 in my case. Second, we want to find EFI partition for Pop!_OS; that is nvme0n1p5 for me.

Be sure to replace those values with what you have in future commands!

Mount the Windows EFI Partition

Let's do the following commands to mount the EFI partition and inspect it.

In my case, I'd put nvme0n1p1 for the Windows EFI partition.

Then, we want to make sure that the Windows bootloader exists before we can proceed.

Create a Windows Entry for systemd-boot

We're going to create a file in /boot/efi/loader/entries/ for our Windows entry in the bootloader.

Paste the following into the file (file should be currently empty).

Again, In my case, I'd put nvme0n1p1 for the Windows EFI partition.

Note: If you've followed through with the rest, and you're still not seeing Windows listed in the bootloader options after you restart, try the below steps. If this is your first time going through this, skip this part.

The last command is to make sure the boot file exists. Again, this part is only if the previous steps didn't work. Meaning you have tried all the other instructions other than this, and you still didn't see Windows listed in the bootloader options.

Cleanup

We now need to unmount Windows EFI.

Configure systemd-boot Menu

We're now going to edit the loader config file to add a timeout.

You should only see one line currently in this file. Add a new line, and paste the below content.

Update systemd-boot

Now we're just going to update, and check if we indeed see Windows listed.

After that last command, you should see entries like these listed.

Finished

That's all! From here, you should be able to reboot, see the bootloader menu come up, and see Pop!_OS and Windows listed as options!