Well at the place of OpenSSL … you have to put actually the path to your OpenSSL folder that you have downloaded. Your actual command should look like this:
keytool -exportcert -alias androiddebugkey -keystore ~/.android/debug.keystore | "C:\Users\abc\openssl\bin\openssl.exe" sha1 -binary | "C:\Users\abc\openssl\bin\openssl.exe" base64
Remember, the path that you will enter will be the path where you have installed the OpenSSL.
Edit:
you can download OpenSSL for windows 32 and 64 bit from the respective links below: