That no problem of the string type but the IDE, you can write it like this e.g.
Const
C_URL = 'A long text with 255 Characters ....to be contionued ...'
+'more content....to be contionued ... '
+'more more content....to be contionued ... '
+'enough content';
begin
IDHTTP1.Get(C_URL);
end;
or
IDHTTP1.Get( 'A long text with 255 Characters ....to be contionued ...'
+'more content....to be contionued ... '
+'more more content....to be contionued ... '
+'enough content');