How to assign an initial value to a bit in a vector while at the same time declaring an alias for it?

Question

I have a vector called ctst,

signal ctst    : std_logic_vector(19 downto 0);

and I am trying to assign an initial value to each bit of the below signals:

alias SS_DELAY : std_logic_vector(3 downto 0) is ctst(19 downto 16);
alias CPOL     : std_logic is ctst(15);
alias WB_MODE  : std_logic is ctst(14);
alias ASS      : std_logic is ctst(13);
alias IE       : std_logic is ctst(12);
alias LSB      : std_logic is ctst(11);
alias Tx_NEG   : std_logic is ctst(10);
alias Rx_NEG   : std_logic is ctst(9);
alias GO_BSY   : std_logic is ctst(8);
alias CHAR_LEN : std_logic_vector(6 downto 0) is ctst(6 downto 0) ;

The issue when I am trying to assign for example a signal equal to '0',

alias ASS      : std_logic is ctst(13) := '0';

it gives me this error

Error (10500): VHDL syntax error at wb_spicore_gpio.vhd(127) near text ":=";  expecting "(", or "'", or "."

My main point is to assign an initial value for each one of the above signals.

How can I do this?