Astra Theme - Add 'Apply Now` button in the Header conditionally with a link to page with a form to apply.

dynamic-astra-customizer-options-update.php

<?php // don't copy this line in your file.

// Condition to check if this page requires `Apply Now` button.
if ( 1481 === get_the_id() ) {
	your_prefix_setup_header_button_apply_now();
}

/**
 * Setup filters for changing the values in the Astra Settings.
 */
function your_prefix_setup_header_button_apply_now() {
	add_filter( 'astra_get_option_header-main-rt-section', 'your_prefix_header_rt_text_html' );
	add_filter( 'astra_get_option_header-main-rt-section-html', 'your_prefix_header_rt_text_html_content' );
}

/**
 * Returns the item key for text-html area in the custom menu item.
 * 
 * @return String option key for text-html area in the custom menu item.
 */
function your_prefix_header_rt_text_html() {
	return 'text-html';
}

/**
 * Returns markup required for the `Apply Now` button.
 * 
 * This can be any HTML markup of your choice.
 * Here 1792 is the ID for the page where the `Apply Now` form is.
 * 
 * @return String HTML markup for the `Apply Now` button.
 */
function your_prefix_header_rt_text_html_content() {
	return 
	'<span class="top-nav-book-cta">
		<a href="'. get_permalink( 1702 ) .'">Apply Now</a>
	</span>';
}