How to assign value to argument column name?

declare 
    ll_maxid integer;
    arg_column text :=TG_ARGV[0]::text;
begin

    if TG_WHEN ='BEFORE' AND TG_OP='INSERT' THEN
        
        execute format('SELECT coalesce(max(%s),0)+1 FROM %I.%I',arg_column,TG_TABLE_SCHEMA, TG_TABLE_NAME) INTO ll_maxid;
        
        new.arg_column := ll_maxid;
        
    end if;
    return new;
    
end;

Error - The record new has no field'arg_column'

Give me solution...