Shortcut to Bottom border in Numbers
Custom shortcuts only work if there is a menu item that matches the command.  As there is no Border bottom menu command, this may not be possible.

Borders can be of any style and weight, so even if it were possible, it seems that such a shortcut would format the bottom border that was last copied.

A work around would be to format a cell with the bottom border of your choice, then copy the formatting only.  Format > Copy Style or option+command+C

Then use option+command+V to Paste the style to a newly selected cell.

DEMO:



Note that your custom key combination as pictured in the question conflicts with the built in shortcut for Paste Style.