Below is a quick way to find your startup program that’s called after a reboot.  Copy that program, which is a CL program, to your source file and edit it in PDM to add the command to startup the Zend Server.  You then compile the CL program and point the system value for the startup program to that newly compiled program.

#Find where your startup program is. (Program called after an IPL (Reboot))
DSPSYSVAL SYSVAL(QSTRUPPGM)
#Create a source file
CRTSRCPF FILE(MYLIB/MYFILE)
#Put the startup Program in that file
RTVCLSRC PGM(SOMELIB/THESTARTUPPGM) SRCFILE(MYLIB/MYFILE) SRCMBR(MYPROGM)
STRPDM
#Option 3 work with members
LIB: MYLIB
FILE: MYFILE
#Option 2 Edit
#Add this to your source file
STRTCPSVR SERVER(*HTTP) HTTPSVR(ZENDSVR)
#Creat the CL Program
CRTCLPGM PGM(MYLIB/MYFILE) SRCFILE(MYLIB/MYFILE)
#Point the system value of the startup program to the new program
CHGSYSVAL SYSVAL(QSTRUPPGM) VALUE('MYLIB/MYFILE')

Shout out to @gmantechi for the clues

 

References

https://publib.boulder.ibm.com/iseries/v5r2/ic2924/info/rzal2/rzal2chgiplstartup.htm

http://publib.boulder.ibm.com/html/as400/v4r5/ic2931/info/db2/rbal1mst106.htm