# Mount root btrfs to create swap subvolume
sudo mount /dev/mapper/your-btrfs-decrypted-device /mnt
sudo btrfs subvol create /mnt/swap
sudo chmod 700 /mnt/swap

# Create mountpoint for the subvolume that will contain the swapfile
sudo mkdir /swap
sudo mount -o subvol=swap /dev/mapper/your-btrfs-decrypted-device /swap

# Create the swapfile and set required parameters
sudo truncate -s 0 /swap/swapfile
sudo chattr +C /swap/swapfile
sudo btrfs property set /swap/swapfile compression none
# Make the swapfile 20 gigabytes, change this to whatever
sudo fallocate -l 20G /swap/swapfile
sudo chmod 600 /swap/swapfile
sudo mkswap /swap/swapfile

# Now that you have a swapfile ready, you need to find the location of it using the special btrfs program
# I assume you've compiled it already as described:
# https://wiki.archlinux.org/title/Power_management/Suspend_and_hibernate#Hibernation_into_swap_file_on_Btrfs
./btrfs_map_physical /swap/swapfile

So now you get a number as described, you need to multiply that number by pagesize to get the final offset, after that, here's my guix configuration:
(operating-system
 (kernel-arguments
  '("resume=/dev/mapper/your-decrypted-btrfs-name"
    "resume_offset=<that number you calculated above>"))
 (file-systems
  (cons*
    (file-system
      (mount-point "/")
      (device "/dev/mapper/your-btrfs-device")
      (flags '(no-atime))
      (options "subvol=swap,nodatacow,nospace-cache")
      (type "btrfs")
      (check? #f)
      (create-mount-point? #t)
      (needed-for-boot? #f)
      (dependencies mapped-devices))
    ... your other filesystem declarations
    %base-file-systems))
 (swap-devices
  (list
    (swap-space
      (target "/swap/swapfile")
      (dependencies file-systems)))))