If you have to select F8, then the PXE server doesn't think you have anything for it in automation. After selecting F12, if you've configured everything correctly, the rest should "just happen" without intervention.
For instance, if you've enabled "initial deployment" then any "unknown" computers will automatically boot to that menu and do that work. However, any "known" computers will not. They will boot to PXE, and try to go to production because there's no work to be done. If you FORCE it to go to PXE, once the agent checks in and finds no work to be done, it will try to go to production.
In short, you have to give the computer instructions to go to PXE. If the system is "known" already to the console, you have to create at LEAST a bogus job to boot to pxe and "wait" - which we have some jobs for. Preferrably, you create a job to go to PXE and do something.
If it's an unknown computer, then you have to ensure Initial Deployment is configured correctly, so that you don't have to press F8 to get to that menu. Remember though that Initial Deployment only works for unknown computers, not known ones!