Use ATEN KVM switch like KH1516A, through KA7169 adapter to manage my DELL PC Optiplex 5050/7050.

When system boot up , the BIOS post screen just appear several seconds, then enter to the windows desktop immediately.

I am unable to press the keyboard button in time to enter BIOS setup menu.

This symptom is caused by the BIOS behavior, not related to KVM switch.

Some newer PCs provide an option setting in BIOS -> extend BIOS POST time. 

This option create an additional delay time allows the user to see POST message.

Please change the POST time setting to 10 seconds.