I replaced ecm with a used one . How do I program key to operate the new ecm ? The car will not turn over

You have to reprogram the new PCM with the Ford scantool, performing a PMI or "programmable module installation." The info from the original PCM is downloaded into the scantool, then programmed from the scantool into the replacement PCM. You'll either need the scantool and knowhow to do the swap or let a shop with the scantool do it.

Is there any other way to make existing key i have work with replacement computer. A way to program key without scan tool

No, the new PCM needs to be programmed to accept the old keys.

What happens if you don't have the old PCM

Where in south east king county I have a 1999 ford sho with a new pcm it will not start . I would get it to some place to get the new pcm programed to use old key to start car.

King county is in state of Washington st.