Explicitly set input focus to the window.
int SDL_SetWindowInputFocus(SDL_Window * window);
the window that should get the input focus
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
You almost certainly want SDL_RaiseWindow() instead of this function. Use this with caution, as you might give focus to a window that is completely obscured by other windows.
This function is available since SDL 2.0.5.