If I understand you correctly, and you want to exit from LoginBtn_Clicked without having to check the return value of CheckPassword and 'returning' - well, you can't.
You could do what you originally had, or this:
void LoginBtn_Clicked()
{
if (CheckPassword("xxx"))
{
}
}
Or, you could get CheckPassword to throw an exception, but that's an awful awful solution. Why do you want it to be void? It absolutely should return a bool, because the password check either fails or succeeds.