A script can simulate keystrokes/mouse clicks on Windows. I believe the UAC prompt is protected by the OS making automating clicking it a more difficult task. (I'm not a windows user; but its straightforward to simulate mouse clicks/movement in windows). Apparently the UAC screen is run as the System user (a user with more permissions than an administrator, but not quite the kernel) so unless you were able to write and execute a script as the System user you would not be able to bypass the UAC clicks.
But in general, having a GUI login with a button doesn't make your login screen any more difficult to break. Any reasonable attacker will examine how your program is checking the password, extract out the saved hash from memory and write some massively parallel GPU code to brute force it. (Unless its done over the network and then they'd stop using your GUI and do the requests directly). But even if somehow constrained to simulating keystrokes and mouse movements/clicks its still trivial to do.