Install this version:
emerge -a =acct-user/matrix-webhook-0-r1
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =acct-user/matrix-webhook-0-r1
Or alternatively:
emerge --autounmask-write -a =acct-user/matrix-webhook-0-r1
# Copyright 1999-2022 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
inherit acct-user
KEYWORDS="~amd64 ~x86"
ACCT_USER_ID=-1
ACCT_USER_GROUPS=(${PN})
ACCT_USER_HOME=/var/lib/${PN}
acct-user_add_deps