PL/PGSQL - How to pass in variables?

Using PL/PGSQL, I am trying to create a procedure to display the
count of rows in any single table of a database. The End-user would
pass in a table name and the prodecure would display the table name
with the row count.

I am able to hardcode the variable for table and get the appropriate
results from my count function (see below), but cannot pass in a
variable and have the function work. Any suggesstions???
 
CREATE FUNCTION get_table_count(tablename text) RETURNS integer AS $$
DECLARE
     
     --tablename ALIAS FOR $1;
     
      rowcount INTEGER;
   BEGIN
     

     SELECT INTO rowcount count(*) FROM tablename;
     
     RETURN rowcount;
     
   END;
 $$ LANGUAGE 'plpgsql';