You should be able nowadays with PG16

    INSERT INTO tbl1 VALUES ($1, $2) \bind 'first value' 'second value' \g

https://www.postgresql.org/docs/current/app-psql.html#APP-PS...

For older versions, you can do:

    \set v_x 'first value'
    \set v_y 'second value'
    INSERT INTO tbl1 VALUES (:'v_x', :'v_y');
    \set v_x 'next value'
    INSERT INTO tbl1 VALUES (:'v_x', :'v_y');