What's the usual agent added to discourage drinking of hazardous substances
It is denatonium benzoate. This comes from wikipedia page of denatonium:

Denatonium, usually available as denatonium benzoate (under trade
names such as Denatrol, BITTERANT-b, BITTER+PLUS, Bitrex or Aversion)
and as denatonium saccharide (BITTERANT-s), is the most bitter
chemical compound known, with bitterness thresholds of 0.05 ppm for
the benzoate and 0.01 ppm for the saccharide. It was discovered in
1958 during research on local anesthetics by MacFarlan Smith of
Edinburgh, Scotland, and registered under the trademark Bitrex.

With respect to discouraging drinking of hazardous substances it again comes from wiki:

The bitterness of the compound guides most applications of denatonium.
Denatonium benzoate is used to denature ethanol so that it is not
treated as an alcoholic beverage with respect to taxation and sales
restrictions.