If you need to 301 redirect all the traffic from olddomain.com to the index page of newdomain.com then add the following code to your root .htaccess page on olddomain.com
########################
# 301 redirect all traffic from domain A to the root of domain B
# See: https://www.jucra.com/whmcs/knowledgebase/76/
########################
Options +FollowSymLinks
RewriteEngine on
RewriteRule (.*) https://www.newdomain.com [R=301,L]Any traffic from any part of olddomain.com will now go to the home page of newdomain.com