Ever since pihole-FTL was introduced with Pi-hole v4 in 2018, dnsmasq has not been used for DHCP, but note that pihole-FTL is a tailored fork of dnsmasq using the same configuration options, except for those configuration options specific to pihole-FTL.
Your post is not quite clear, but it seems you are looking for a way to apply some custom dnsmasq configuration options you've been using with Pi-hole v5 to Pi-hole v6?
If that's the case, hasn't that been answered already?
If you need to add custom dnsmasq configuration details that are not covered by pihole.toml, you may add them as described in the aforementioned /etc/pihole/dnsmasq.conf:
# IF YOU WISH TO CHANGE ANY OF THESE VALUES, CHANGE THEM IN #
# /etc/pihole/pihole.toml #
# and restart pihole-FTL #
# #
# ANY OTHER CHANGES SHOULD BE MADE IN A SEPARATE CONFIG FILE #
# WITHIN /etc/dnsmasq.d/yourname.conf #
# (make sure misc.etc_dnsmasq_d is set to true in /etc/pihole/pihole.toml) #