@Hubi i’m pretty sure there’s references on how to do it on the forums, but here’s how i’ve done it in the advanced menu. it will take just a little modification to put this into the ipxe menu entries instead, but it should give you a pretty good idea of where to start

:Win7SP1x64 kernel bzImage root=/dev/ram0 rw ramdisk_size=127000 ip=dhcp web=${fog-ip}/fog/ consoleblank=0 loglevel=4 type=down img=Win7SP1x64 ftp=${fog-ip} imgType=n imgPartitionType=all osid=5 storage=${fog-ip}:/images/ capone=1 imgfetch init.xz boot || goto MENU