A simple R1-C-R2 circuit should do. Button is isolated from the input via a small cap and a pull-up resistor at each side of the cap discharges it when the button is released. When pressed, the other side of the cap will go low for whatever RC time selected (R2) and then rise. If the input is sensitive, add a protective diode (cathode to positive side, anode to input).
If you need the pulse to go high and rest low, just use pull-downs instead and move the eventual diode so cathode goes to input and anode goes to 0V.