Use
ffmpeg -y -rtbufsize 100M -f gdigrab -framerate 20 -offset_x 0 -offset_y 0
-video_size 1250x750 -draw_mouse 1 -i desktop
-f dshow -i video="HP Truevision HD":audio="Microphone (High Definition Audio Device)"
-filter_complex "[0]pad=height=ih+10:color=black[b];
[b][1:v]overlay=(main_w-overlay_w):main_h-overlay_h[v]" -map "[v]"
-c:v libx264 -r 20 -preset ultrafast -tune zerolatency -crf 30 -pix_fmt yuv420p
-map 1:a -c:a libmp3lame -qscale:a 4 "D:\today.mp4"