[Question] How to do a GUI in internal(.dll) application?

[Walrus]

Hi!++
Is there a way to do a GUI in .dll file? It mean: I want do a .dll file, inject it to process (for example firefox) then there should be a GUI, but I don't know how to do it. I'm using QT creator as IDE and want do it in c++

Like it:

void tGui()
{
//the qt instruction (creating and showing window)
}
BOOL APIENTRY DllMain(HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved)
{
switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
CreateThread(nullptr, 0, tGui, nullptr, 0, nullptr);
break;
}
return TRUE;
}

void tGui()
{
//the qt instruction (creating and showing window)
}
BOOL APIENTRY DllMain(HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved)
{
switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
CreateThread(nullptr, 0, tGui, nullptr, 0, nullptr);
break;
}
return TRUE;
}

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I also don't see a option to create .dll file in QT creator :/
Thanks for help