Automatic activation of ACF license via PHP. The code adds the key to the option in the desired format, and adds the domain to the list of trusted domains tracked in the ACF profile, so that the update is successful. The code can be used as a MU plugin, a regular plugin, or inserted into the theme code.

set_acf_pro_license.php

<?php

add_action( 'acf/init', 'set_acf_pro_license' );

function set_acf_pro_license() {
	global $acf_instances;

	if ( isset( $acf_instances['ACF_Admin_Updates'] ) && ! get_option( 'acf_pro_license' ) ) {
		/**
		 * @var ACF_Admin_Updates $acf
		 */
		$acf = $acf_instances['ACF_Admin_Updates'];

		$_POST['acf_pro_licence'] = 'YOUR_LICENCE_KEY';

		$acf->activate_pro_licence();
	}
}