Modify the text of the login link in all membership checkout confirmation emails.

my_pmpro_email_custom_login_link_text.php

<?php
/**
 * Modify the text of the login link in all membership checkout confirmation emails.
 *
 */
function my_pmpro_email_custom_login_link_text( $body, $email ) {
	// Only filter checkout emails.
	if ( strpos( $email->template, 'checkout' ) !== false ) {
		// Change text "Log in to your membership account here:" to "Log in to get started:".
		$body = str_replace( 'your membership account here', 'get started', $body );
	}
	return $body;
}
add_filter( 'pmpro_email_body', 'my_pmpro_email_custom_login_link_text', 10, 2 );

This recipe is included in the blog post on "Filtering PMPro Emails" at Paid Memberships Pro here: https://www.paidmembershipspro.com/filtering-pmpro-emails/