How to make recorded mp4 file constant frame rate and variable bit rate?

I am using CaptureEngine video capture sample from:

https://github.com/microsoftarchive/msdn-code-gallery-microsoft/tree/master/Official%20Windows%20Platform%20Sample/CaptureEngine%20video%20capture%20sample/%5BC%2B%2B%5D-CaptureEngine%20video%20capture%20sample/C%2B%2B

The frame rate of recorded mp4 file (H264/AAC) is variable and the bit rate is constant. Could anyone advice how to change the configuration/settings to make the recorded file as constant frame rate and variable bit rate?