RB2018
I would like to save ONLY this midi track. How can I do?

Just drag the track number up to DROP MID then drag it out to a folder when green.

if a drag the track where BIAB save the file?

Should go into your x:\RealBand\DragDrop\ folder

Another option:

Select the track, click Render and then 'Save Track to file'.

This selects the whole track for saving. Your next decision is what format to save it to.
Since you seem to want to save the MIDI track, when the Save dialog opens, make sure to set the file type to MID (it will likely default to SEQ format).

Then choose your desired location and just save it.

When Rharv said "select a track" what he meant to say was "use Ctrl + A to select the whole track".

Selecting all of a track is not obvious when the timeline extends beyond the monitor. That's one keyboard shortcut well worth remembering in RealBand.

Actually, no

If you use the path I described, the prompts imply it should select the whole track ..
In my tests I can have 1-1-0 thru 4-1-0 selected in the From-Thru, but using the 'Render-Save track to file' path it selects the whole track (does CTRL+A automagically) and then saves the whole MIDI track to its own file.

At least it does here
In the image below, I clearly have a smaller section of an audio track selected, (in the From-Thru)
But I have a different MIDI track selected to Save to File.
The pop up says it will select the track (implying the whole track) and save it .. which it does. In my result I get the whole MIDI track saved to a file without having to adjust the From-Thru (or CTRL+A).

Maybe others have different results but that's how it works here.

Although CTRL+A is well worth noting/remembering ..