My ex partner has established our web application’s Cloudflare account to his own Cloudflare account. I have no contact with him anymore. Whar can I do to get my application on Cloudflare with my account?
You would need to add the domain to your own account and reconfigure it there, then change the nameservers at the domain registrar to point to your account.
If the domain is registered at Cloudflare then you may not be able to gain access to it or have to go the legal route if you own the trademark etc.