plpgsql.. SELECT INTO ... WHERE FIELD LIKE

How in plpgsql use LIKE with a variable?

let say I want to do this query:

SELECT INTO RS id FROM customer WHERE firstname LIKE keyword% LIMIT 1;

keyword is a variable, in this case I want to find name like 'Jo%'

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Full function:

CREATE OR REPLACE FUNCTION custlike(text) RETURNS INT4 AS'

DECLARE
keyword ALIAS FOR $1;
RS RECORD;

BEGIN
SELECT INTO RS id FROM customer WHERE firstname like keyword% LIMIT 1;
IF FOUND THEN
RETURN RS.id;
ELSE
RETURN NULL;
END IF;

END'
LANGUAGE 'PLPGSQL';