EDIT: some modifications according to rodrigo answer in comments.
INPUT Input = { 0 };
// shift key down
Input.type = INPUT_KEYBOARD;
Input.ki.wVk = VK_LSHIFT;
SendInput( 1, &Input, sizeof( INPUT ) );
// 'a' key down
Input.type = INPUT_KEYBOARD;
Input.ki.wVk = 'A';
SendInput( 1, &Input, sizeof( INPUT ) );
// 'a' key release
Input.type = INPUT_KEYBOARD;
Input.ki.dwFlags = KEYEVENTF_KEYUP;
Input.ki.wVk = 'A';
SendInput( 1, &Input, sizeof( INPUT ) );
// shift key release
Input.type = INPUT_KEYBOARD;
Input.ki.dwFlags = KEYEVENTF_KEYUP;
Input.ki.wVk = VK_LSHIFT;
SendInput( 1, &Input, sizeof( INPUT ) );
EDIT: here is another code with an example of turning caps/shift off after sending letter:
INPUT Event = { 0 };
const SHORT key = VkKeyScan('a');
const UINT mappedKey = MapVirtualKey( LOBYTE( key ), 0 );
// Press shift key
Event.type = INPUT_KEYBOARD;
Event.ki.dwFlags = KEYEVENTF_SCANCODE;
Event.ki.wScan = MapVirtualKey( VK_LSHIFT, 0 );
SendInput( 1, &Event, sizeof( Event ) );
// upper case 'A'
Event.type = INPUT_KEYBOARD;
Event.ki.dwFlags = KEYEVENTF_SCANCODE;
Event.ki.wScan = mappedKey;
SendInput( 1, &Event, sizeof( Event ) );
// release upper case 'A'
Event.type = INPUT_KEYBOARD;
Event.ki.dwFlags = KEYEVENTF_SCANCODE | KEYEVENTF_KEYUP;
Event.ki.wScan = mappedKey;
SendInput( 1, &Event, sizeof( Event ) );
// Release shift key
Event.type = INPUT_KEYBOARD;
Event.ki.dwFlags = KEYEVENTF_SCANCODE | KEYEVENTF_KEYUP;
Event.ki.wScan = MapVirtualKey( VK_LSHIFT, 0 );
SendInput( 1, &Event, sizeof( Event ) );
const SHORT key1 = VkKeyScan('A');
const UINT mappedKey1 = MapVirtualKey( LOBYTE( key1 ), 0 );
// lower case 'a'
Event.type = INPUT_KEYBOARD;
Event.ki.dwFlags = KEYEVENTF_SCANCODE;
Event.ki.wScan = mappedKey1;
SendInput( 1, &Event, sizeof( Event ) );
// release lower case 'a'
Event.type = INPUT_KEYBOARD;
Event.ki.dwFlags = KEYEVENTF_SCANCODE | KEYEVENTF_KEYUP;
Event.ki.wScan = mappedKey1;
SendInput( 1, &Event, sizeof( Event ) );
explanation here (I hope the explanation is correct): SendInput() for keyboard - only uppercase
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