Now I'm confused. YOU told us:
...
This is not the Reaper support forum 
Anyway:
To add a "User Defined" TXXX:BPM
File -> Project Render Data
Double Click on the empty value column at the row "User Defined"
Enter the Name: BPM
Enter =
Enter the value: 123
Press OK
You have to "render" the music track to get this metadata written.
(At least I have not yet found any other way to write this User Defined metadata directly and immediately in the fields of an existing track with Reaper.)
The official BPM tag (TBPM) can be filled much more easily:
Just right click on the track in the Media Explorer.
Click on "Edit Metadata tag"
Choose BPM
Enter the value and press OK
After a click on
"Write edited Metadata to media"
you will find a TBPM with the entered value.