Struggling with SendInput()

I want to inject every possible combination of 4 digit numbers into a different program (eg notepad) by imitating the keyboard, but I cannot for the life of me figure out how to use the SendInput() function, or if there is a better alternative.

string code;
for (int digit1 = startThousand; digit1 < 10; digit1++){
    for (int digit2 = 0; digit2 < 10; digit2++){
        for (int digit3 = 0; digit3 < 10; digit3++){
            for (int digit4 = 0; digit4 < 10; digit4++){

            code = (std::to_string(digit1)+std::to_string(digit2)+std::to_string(digit3)+std::to_string(digit4));
                    //generating the 4 digit codes and outputting the final code into a single variable
            cout << code << "\n";
                    //printing the code
            SendInput(code, ???, sizeof(code))
                    //sending the code into a different program

}}}}

SendInput does not take in a string as parameter.

  Sendinput(cInputs, pInputs, cbSize);

//cInputs = number of structures in the pInputs array..
//pInputs = a pointer to a INPUT structure which contains info about your mouse/key event. 
//cbSize  = the size of the structure. Tip! use sizeof();

//Step 1.
//Declare a KEYBDINPUT struct and set appropriate values to each variable.
//See MSDN...
//
//Step 2.
//Declare a INPUT structure.
//Set type: INPUT_KEYBOARD. then set "yourinputstruct".ki equal to your keybdinput struct
//
//Step 3:
//Use Sendinput function! Sendinput(1,&yourinputstruct, sizeof(yourinutstruct));

for list of Virtual Keycodes see MSDN. https://learn.microsoft.com/en-us/windows/win32/inputdev/virtual-key-codes

Here is the function that I wrote, it still may need some improvements but I think it will do what you are asking for! at least it´s working for me typing in cmd and notepad... Read on msdn or ask me to comment if it is hard to understand!

Just pass in a Virtual keycode as first parameter and true if it is a extended key and false if it´s not!

void KeyPress(unsigned short VK, bool ExtendedKey)
{
    KEYBDINPUT  KeyDown = { 0 };
    KEYBDINPUT  KeyUp = { 0 };
    INPUT Input[2] = { 0 };

    //KeyDown...
    KeyDown.wVk = VK;
    KeyDown.wScan = 0;
    KeyDown.time = 10;
    KeyDown.dwFlags = 0;
    KeyDown.dwExtraInfo = 0;

    if (ExtendedKey == true)
    {
        KeyDown.wVk = VK;
        KeyDown.wScan = 0;
        KeyDown.time = 1000;
        KeyDown.dwFlags = KEYEVENTF_EXTENDEDKEY;
        KeyDown.dwExtraInfo = 0;
    }

    //KeyUp...
    KeyUp.wVk = VK;
    KeyUp.wScan = 0;
    KeyUp.time = 10;
    KeyUp.dwFlags = KEYEVENTF_KEYUP;
    KeyUp.dwExtraInfo = 0;

    if (ExtendedKey == true)
    {
        KeyUp.wVk = VK;
        KeyUp.wScan = 0;
        KeyUp.time = 10;
        KeyUp.dwFlags = KEYEVENTF_EXTENDEDKEY | KEYEVENTF_KEYUP;
        KeyUp.dwExtraInfo = 0;
    }
    //Setup Input...
    Input[0].type = INPUT_KEYBOARD;
    Input[0].ki = KeyDown;

    Input[1].type = INPUT_KEYBOARD;
    Input[1].ki = KeyUp;

    //Click and Release!
    SendInput(1, &Input[0], sizeof(Input[0]));
    SendInput(1, &Input[1], sizeof(Input[1]));
}