Program doesn't work after it loops the second time - c

Question

My program below is trying to add the usernames from the "Usernames.text" to the API url. The first username works and makes the url "https://api.1234ng.com/users/profiles/planeraft/Username1" but once it loops back it makes the url always "https://api.1234ng.com/users/profiles/planeraft/" instead of adding the next username in the file.
My code:
#include <stdio.h>
#include <string.h>
int Counter;
int UsernameSize;
char Username[255];
char Request[255]="https://api.1234ng.com/users/profiles/planeraft/";
FILE *UsernamesFile;
int main() {
UsernamesFile = fopen("Usernames.text", "r");
while (Counter < 100) {
fgets(Username, 255, UsernamesFile);
UsernameSize = strlen(Username);
for (int z; z < UsernameSize; z++) {
Request[48 + z] = Username[z];
}
printf("%s", "\n");
printf(Request);
for (int a; a < UsernameSize; a++) {
Request[48 + a] = '\0';
}
Counter++;
}
fclose(UsernamesFile);
return 0;
}

Answer 1

Variables a and z in the for loops are not initialized and thanks to DiKetarogg for some improvements to the code.

Answer 2

Several errors fixed below, more improvements are possible, see comments above and in code below:
#include <stdio.h>
#include <string.h>
int Counter = 0; // need to init
int UsernameSize;
char Username[255];
char Request[255]="https://api.test.com/users/profiles/test/";
// 01234567890123456789012345678901234567890
// 1 2 3 4
FILE *UsernamesFile;
int main() {
UsernamesFile = fopen("Usernames.text","r");
while(Counter < 3) {
fgets(Username, 255, UsernamesFile);
strcat(Request, Username);
UsernameSize = strlen(Username);
Username[UsernameSize-1] = '\0'; // remove <cr>
for(int z=0; z < UsernameSize; z++) {
Request[41+z]=Username[z];
}
printf("%s\n", Request);
Counter++;
strcpy(Request, "https://api.test.com/users/profiles/test/");
}
fclose(UsernamesFile);
return 0;
}