Cannot set placeholder text via shortcode

Hello. I am attempting to set the placeholder text via the shortcode attributes, but it is being ignored.

Please take a look at the efforts below and let me know if anything is wrong.

(Since I need to store and display “affiliation” in addition to name, I am trying to use the placeholder text to convey that the full name should go in the first name field, and the affiliation should go in the last name field.)

First I tried this:

[lh_signing_form 'first_name_placeholder' => 'Your Name', 'last_name_placeholder' => 'Your Affiliation', 'email_placeholder' => 'Your Email']

Then I tried this:

[lh_signing_form 'first_name' => 1 , 'first_name_placeholder' => 'Your Name', 'last_name' => 1, 'last_name_placeholder' => 'Your Affiliation', 'email_placeholder' => 'Your Email']

The shortcode settings were ignored – the page displayed with the default values of placeholder texts.