how to fill pointer element of struct with array in C

First of all your pData must point to some valid buffer if it points to some valid buffer you can fill it just like that.

for(i=0; i<MAX; i++) {
    // Note that newData must be this same type or you have
    // to truncate whatever type you are writing there to char.
    myTag.pData[i] = newData[i];
}

If you want to copy this same value all over the buffer which I doubt (however your code implies that) just use memset standard library function for this purpose.

Please specify on the example what do you want to happen to this buffer and give more context of your use case. It looks like you are beginner and you might need a little bit more help than you asked in order to make your program fine.

Here you have working code example for sth I think you want but you don't know how to ask about it.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct tag{ char *pData; };

int main(void) {
    int i;
    struct tag myTag;
    myTag.pData = (char*)malloc(10*sizeof(char));
    if(NULL == myTag.pData) {
        return 0;
    }

    const char *test = "Hello";

    for(i=0; i<strlen(test); i++) {
        myTag.pData[i] = test[i];
    }

    // Put null termination at the end of string.
    myTag.pData[i] = '\0';

    printf("%s", myTag.pData);

    free(myTag.pData);

    return 0;
}