It looks like working. Thank you for your help.
Just in case here my full /etc/init.d/beszel-agent script
#!/bin/sh /etc/rc.common
START=99
USE_PROCD=1
# Config
PROG=/usr/bin/beszel-agent
KEY="ssh- key copy from beszel"
ADDR="192.168.1.1:45876"
#EXTRA_DISK=""
start_service() {
procd_open_instance
procd_set_param command "$PROG"
# Parameters
procd_append_param command -key "$KEY"
procd_append_param command -l "$ADDR"
if [ -n "$EXTRA_DISK" ]; then
procd_append_param command -ef "$EXTRA_DISK"
fi
# Enable in AppArmor the MPTCP
procd_set_param env GODEBUG=multipathtcp=0
# Automatic restart
procd_set_param respawn ${respawn_threshold:-3600} ${respawn_timeout:-5} ${respawn_retry:-5}
# Logging
procd_set_param stdout 1
procd_set_param stderr 1
procd_close_instance
}