convert &apex_user. from pki to an id from a table

APEX 19 using PKI authentication.

need to convert the global variable &app_user. to a user_code user_id

this doesnt work: "select user_code into user_id from user_table where pki_certificate=&app_user.; "

desc user_table: pki_certificate (unique allow null) and the user_code (unique not null)

the new local variable user_id will be used to display that users data. ex. select * from data_table where owner=user_id;

right now I have a button with the pki cert (&app_user.)

now just want a button to display the user_id