X-AUTH is not deprecated from 15.1X49-D80 and you need to use "aaa" profile. Below given is a sample config.
set security ike gateway remote-vpn1 ike-policy ike-pol2
set security ike gateway remote-vpn1 dynamic hostname "user1@juniper.net"
set security ike gateway remote-vpn1 dynamic connections-limit 2
set security ike gateway remote-vpn1 dynamic ike-user-type shared-ike-id
set security ike gateway remote-vpn1 external-interface ge-0/0/1
set security ike gateway remote-vpn1 aaa access-profile aaa-prof1
set security ike gateway remote-vpn1 version v1-only
set access profile aaa-prof1 authentication-order password
set access profile aaa-prof1 client test firewall-user password "$9$N5VsgGDkTz6oJz69A1INdb"
set access profile aaa-prof1 address-assignment pool xauth-pool
set access address-assignment pool xauth-pool family inet network 10.1.1.0/24
set access address-assignment pool xauth-pool family inet xauth-attributes primary-dns 10.219.194.50/32
Please note that from 15.1X49-D80 , NCP is the official supported remote VPN client for SRX.