How to format [SearchBy] field for a single letter

I need to add the first letter of an artist to the [SearchBy] field.

Artist= baton rouge
Title= dident i

This is what I currently have,
which translates to:

I need the search to translate to:
-Adding the value "b" which is the first letter of the artist "baton_rouge".

The "?" is where the string needs to be:

Thanks for any help.