I have seen several ways of doing this depending which PLC platform you are using but I think that the easiest way to do this is by using a simple Count up counter. Have the count increment by one each time you press the P.B. and then compare the current value of the counter, when the value is equal to 1 turn on a coil. You will need to reset the counter and you will also need a one shot to avoid the counter incrementing every PLC scan.
Feel free to e-mail me and I can forward you an example
Best regards,
Don