User input to queries

While I develop the application's GUI I use the database from the command
line (psql). While some queries are self-contained others need user input.
I've not found a search term that locates this information in the
postgres-12 user manual and my web searches suggest that using '&' should
work.

I tried the latter with this query but postgres didn't like it. (There may
well be other errors in it so don't hesitate to point out my mistakes.)

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/* This query selects all activity information for a named person */

/* Specify columns */
SELECT p.lname, p.fname, p.loc_nbr, p.job_title, p.direct_phone, p.active,
o.org_name,
l.loc_nbr, l.loc_name,
a.act_date, a.act_type, a.notes, a.next_contact
/* Specify tables. */
FROM People AS p
JOIN Organizations AS o ON o.org_nbr = p.org_nbr
JOIN Locations AS l ON l.org_nbr = o.org_nbr and l.loc_nbr = p.loc_nbr
JOIN Activities AS a ON a.person_nbr = p.person_nbr
/* Specify rows */
WHERE p.lname = &p.lname AND p.fname = &p.fname;
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Suggestions on what I should read to learn more about this subject are
appreciated.

Regards,

Rich