How can I add an user specific string to act as a key for a simple cypher?

Question

I am modifying this simple qwerty cypher so it accepts a key of the users choosing, can I just add the string to replace the hard coded key or is there more to do?

I have tried using a string from a scanf user input. It does work when the is hard coded in place but I would like to be able to add it as a user.

The original code I am working off is:

char* ciphertext = "qwertyuiopasdfghjklzxcvbnm";    

    char input[500];                                    
    printf("Enter text: ");
    fgets(input, sizeof(input), stdin);                 
    input[strlen(input) - 1] = 0;                       
    int count = strlen(input);                           
    char output[count];                                 
    for(int i = 0; i < count; i++) {                    
        int index = ((int) input[i]) - 97;             
        if(index < 0) {
            output[i] = ' ';                            
        }
        else {
            output[i] = ciphertext[index];

I replaced:

char* ciphertext = "qwertyuiopasdfghjklzxcvbnm";

with:

char KEY[26];                                        
    printf("Enter key text:");
    fgets(KEY, sizeof(KEY), stdin);

As well as swapping ciphertext[index] for KEY[index] in the final line.

My code looks like:

char KEY[26];                                        
    printf("Enter key text:");
    fgets(KEY, sizeof(KEY), stdin);                     

    char input[999];                                    
    printf("Enter text: ");
    fgets(input, sizeof(input), stdin);                 
    input[strlen(input) - 1] = 0;                       
    int count = strlen(input);                          

    char output[count];                                 
    for(int i = 0; i < count; i++) {                     
        int index = ((int) input[i]) - 97;              
        if(index < 0) {
            output[i] = ' ';                            
        }
        else {
            output[i] = KEY[index];                     
        }
    }
    output[count] = 0;                                  

    printf("output: %s\n", output);

The code runs, allows and input for KEY but then skips allowing the user to add a text to encrypt and prints a letter as the encrypted message.

When I use the key: qwertyuiopasdfghjklzxcvbnm it prints d (13th input) when I use: qazwsxedcrfvtgbyhnujmikolp it prints y (16th input) When I use: qazxswedcvfrtgbnhyujmkiolp It prints n (again the 16th input)