On my jammy machine I ran these steps to confirm the --apt-pocket=proposed option was working

lxc launch ubuntu:jammy sru-lp2008026-j
lxc exec sru-lp2008026-j bash
add "deb http://archive.ubuntu.com/ubuntu jammy-proposed main restricted" to /etc/apt/sources.list
sudo apt update
sudo apt-get -t jammy-proposed install autopkgtest
# check autopkgtest version
/home/ubuntu# apt list --installed | grep autopkgtest

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

autopkgtest/kinetic-proposed,now 5.28ubuntu0.22.10.0 all [installed]
exit
# on my machine (building image was too slow in container)
# Building mantic machine to get proposed version of isc-dhcp package
sudo autopkgtest-buildvm-ubuntu-cloud -a amd64 -r mantic -s 15G
lxc file push autopkgtest-mantic-amd64.img sru-lp2008026-j/
# back to container
lxc exec sru-lp2008026-j bash
sudo autopkgtest --setup-commands="apt-get update" --apt-pocket=proposed isc-dhcp -- qemu --ram-size=1536 --cpus 2 ./autopkgtest-mantic-amd64.img

Relevant output from autopkgtest:
Get:1 http://archive.ubuntu.com/ubuntu mantic-proposed/main isc-dhcp 4.4.3-P1-1ubuntu2 (dsc) [2,969 B]
Get:2 http://archive.ubuntu.com/ubuntu mantic-proposed/main isc-dhcp 4.4.3-P1-1ubuntu2 (tar) [10.1 MB]
Get:3 http://archive.ubuntu.com/ubuntu mantic-proposed/main isc-dhcp 4.4.3-P1-1ubuntu2 (asc) [874 B]
Get:4 http://archive.ubuntu.com/ubuntu mantic-proposed/main isc-dhcp 4.4.3-P1-1ubuntu2 (diff) [129 kB]
gpgv: Signature made Thu 11 May 2023 03:13:31 PM BST
gpgv:                using RSA key D98F377EB47CC6DD257A9A07E564B9C275BDD52E
gpgv: Can't check signature: No public key
dpkg-source: warning: cannot verify inline signature for ./isc-dhcp_4.4.3-P1-1ubuntu2.dsc: no acceptable signature found
autopkgtest [12:18:02]: testing package isc-dhcp version 4.4.3-P1-1ubuntu2

rmadison output to match version:
rmadison isc-dhcp
 isc-dhcp | 4.2.4-7ubuntu12          | trusty          | source
 isc-dhcp | 4.2.4-7ubuntu12.12       | trusty-security | source
 isc-dhcp | 4.2.4-7ubuntu12.13       | trusty-updates  | source
 isc-dhcp | 4.3.3-5ubuntu12          | xenial          | source
 isc-dhcp | 4.3.3-5ubuntu12.9        | xenial-security | source
 isc-dhcp | 4.3.3-5ubuntu12.10       | xenial-updates  | source
 isc-dhcp | 4.3.5-3ubuntu7           | bionic          | source
 isc-dhcp | 4.3.5-3ubuntu7.4         | bionic-security | source
 isc-dhcp | 4.3.5-3ubuntu7.4         | bionic-updates  | source
 isc-dhcp | 4.4.1-2.1ubuntu5         | focal           | source
 isc-dhcp | 4.4.1-2.1ubuntu5.20.04.4 | focal-security  | source
 isc-dhcp | 4.4.1-2.1ubuntu5.20.04.5 | focal-updates   | source
 isc-dhcp | 4.4.1-2.3ubuntu2         | jammy           | source
 isc-dhcp | 4.4.1-2.3ubuntu2.3       | jammy-security  | source
 isc-dhcp | 4.4.1-2.3ubuntu2.4       | jammy-updates   | source
 isc-dhcp | 4.4.3-2ubuntu4           | kinetic         | source
 isc-dhcp | 4.4.3-P1-1ubuntu1        | lunar           | source
 isc-dhcp | 4.4.3-P1-1ubuntu1        | mantic          | source
 isc-dhcp | 4.4.3-P1-1ubuntu2        | mantic-proposed | source

We can see that autopkgtest is fetching version 4.4.3-P1-1ubuntu2 of isc-dhcp from mantic-proposed and testing it