To set a bitrate for a video encode in FFmpeg use -b:v which means bitrate for video, then specify the bitrate value. This can be k for kBits or m for megabits, -b:v 2M is 2 Megabits.

An example for a 3 Megabit video bitrate is

ffmpeg -i in.mp4 -b:v 3M out.mp4

As an extra you can also specify the audio bitrate using -b:a

A working example with this in use is

ffmpeg -i in.mp4 -b:v 2M -b:a 124K out.mp4

Using this method is a one pass encode which is not efficient, making using -crf a better approach (see here).