Adding custom form within add_shortcode

whenever I paste the shortcode in the page editor the form posts
automatically and redirects to /wp-admin/admin-post.php with a blank
screen

I don’t know what editor is that (🤔), and I’m not sure if this will fix the above issue, but I noticed your function is using wp_nonce_field() which by default displays/echo the output, so because you’re doing $formHtml .= wp_nonce_field(...), then you need to disable the echo because otherwise you’d run into issues like invalid REST API’s JSON response which then causes the block/Gutenberg editor to fail in saving the post.

// set the 4th parameter to false
$formHtml .= wp_nonce_field('get_job_card', 'get_job_card_nonce', true, false);