” I am trying to scan a user input (ascii character) as an integer, and show the ascii code for the entered character”

What you should do is exact opposite. You should read a character and display it as an integer, i.e.:

char c;
scanf("%c", &c);    // <-- read character
printf("%d", c);    // <-- display its integral value

input: a, output: 97

Also note that while(character != 999) isn’t very lucky choice for a terminating condition of your loop. Checking the return value of scanf to determine whether the reading of character was successful might be more reasonable here:

while (scanf("%c", &character)) {
    printf("ascii: %d\n", character);
}