C Programming: Initialize pointer to char array inside a function

I need help with initializing a pointer to char array inside another function.

I'm passing a char array and pointer to char array to a function. I would like to copy characters from variable 'word' to 'secret' by address/reference/pointer.

I was able to do it by passing values to a function like this:

char    *set_secret(char *word, char *secret)
{
    int     i;

    for (i = 0; word[i] != '\0'; i++)
    {
        if (word[i] == ' ')
            secret[i] = ' ';
        else
            secret[i] = SECRET_SYMBOL;
    }
    secret[i] = '\0';

    return(secret);
}

How can I do to copy characters into the pointer of char array by reference ?

Here's what I have (have checked malloc, it's all good):

SOURCE:

void    init_secret(const char *word, char **secret)
{
    int     i;
    *secret = NULL;
    *secret = (char*)malloc( sizeof(char) * strlen(word) + 1 );

    // trying changing secret value by reference and not by value (copy)
    for (i = 0; word[i] != '\0'; i++)
    {
        if (word[i] == ' ')
            *secret[i] = ' ';
        else
            *secret[i] = '_';
    }
    *secret[i] = '\0';
}

int     main(void)
{
    char    *str = "TALK TO THE HAND";
    char    *secret = NULL;

    init_secret(str, &secret);
    printf("Secret word: %s\n", secret);

    return(0);
}

OUTPUT:

Bus error: 10