How to write a C++ function?

Question

C++ / C#

I'm taking an online course in C++ and I've come across a task that I can't solve and have been scratching my head for a long time.
You need to implement a function that appends a copy of the second string argument to the end of the first string argument. There is enough memory in the first row to store exactly both rows, but no more.
Sample Input:
Hello
World!
Sample Output:
Hello World!
Memory Limit: 256 MB
Time Limit: 5 seconds
how to implement a function using pointers?