Assign track number to discnumber

For a certain audiobook player I use, it would be helpful to make the discnumber the same as the track number. Is there a way to make discnumber = track number?

Use Convert Tag-Tag.

Field: DISCNUMBER
Format String: %track%

from top level menu:

CONVERT > tag to tag
Field:

DISCNUMBER (or is it DISC or DISK or ???? double check what the name of the discnumber field is for your files)

Format String:

%track%

if you want the track number to have leading 0 (e.g., 01, 02, etc.) then use this in format string:
$num(%track%,2)

Or do you want to add the discnumber to the track number so that you get for track 1 on disc 1:
101
and for track 1 on disc 2
201?
then do something like this:
Convert>Tag-Tag for TRACK
%discnumber%$num(%track%,2)