If you ask to configure DHCP in systemd I assume you mean to use systemd-networkd. Then with "automatically" enabled DHCP server you must have a
DHCPServer=yes line in the
[Network] section of your
*.network file. You can add a section
[DHCPServer] with additional options. Here is an example:
rpi ~# cat /etc/systemd/network/12-ap0.network
As you see you can use PoolOffset and PoolSize to define the ip range to be used by the DHCP server.
PoolOffset= takes the offset of the pool from the start of the subnet (192.168.4.0/24, determined by Address=192.168.4.1/24),
PoolSize= takes the number of IP addresses in the pool.
For further information look at
man systemd.network section
Access point as WiFi router/repeater, optional with bridge