If I remember correctly, you cant do that. You are not allowed to change the FQDN in your SaaS hosted ws1 access.
I think you can make a redirect rule from you domain to the SaaS hosted domain.
Please see the question "Can I use DNS C-names?" in the "tips and tricks" section in the following FAQ https://techzone.vmware.com/resource/best-practices-and-faqs-architecting-vmware-workspace-one-acces...