how to adjust the bitrate when exporting an mxf file

Hi 

I have been trying to find a way to export an mxf file (DXCAM EX 35 PAL 50i) with a lower bitrate setting. However, if I choose this preset, there is no way to adjust the bitrate setting.

Does anyone have an solution for this? I have also tried to choose a different DXCAM preset, however, the video height and width can not be kept as 1920 x 1040 as this is what I want.

Thank you.