init warning: Service myservice needs a SELinux domain defined. Please fix

1. you need su to set permissive mode. Or you need source code to disable SELinux, such as disable SELinux in kernel config, or disable SELinux in BOARD_KERNEL_CMDLINE in device/vendor_name/product_name/BoardConfig.mk.

2. if you have the source code, you can define the new domain as you wish.

Please refer to the Android official documents: https://source.android.com/security/selinux/device-policy

section: Label new services and address denials

You have to add the seclabel attribute to the service in your init.rc file, but I don't know if your context will work. I just implemented it myself with the init_exec context:

$ grep yourservice system/sepolicy/file_contexts
/system/bin/vpd u:object_r:init_exec:s0

$ ls -Z path/to/system/bin/yourservice
u:object_r:init_exec:s0 path/to/system/bin/yourservice

$ grep yourservice device/brand/product/init.rc  -A 5
service yourservice /system/bin/yourservice
    seclabel u:r:init:s0
    user root
    group root
    oneshot

Disabling SELinux on Android isn't hard and there are many threads treating the question. Just add either one of the following to your kernel command line parameters (i.e bootargs in U-Boot):

androidboot.selinux=permissive
androidboot.selinux=disabled