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For a legitimate setup, the best option is:
Custom Domain

Make the underlying target URL's service
recognize the visible mask domain as its public address.

Typical steps:
Log into the service that owns the long ugly link.
Look for Custom Domain, Domains, Branding, or White-label URL.
Add:
YourMaskDomain.com
or
www.YourMaskDomain.com

In your domain DNS, add the record they give you, usually something like:
Type: CNAME
Name: www
Value: target-service.example.com

Enable SSL/HTTPS for the domain.
This is the cleanest approach because the browser will show:
https://YourMaskDomain.com
instead of the long target URL.

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Masked Forwarding / Domain Cloaking

Some registrars offer masked forwarding,
sometimes called stealth forwarding.

It keeps YourMaskDomain.com in the address bar
by loading the long URL inside a frame.

Avoid this unless it is just for a very simple landing page,
because it often breaks:
mobile layouts, login pages, forms, sharing links, SEO,
HTTPS/security headers, browser protections like X-Frame-Options, etc.

Also, "cloaking" can mean hiding the real destination from users
or search engines, which can violate platform, ad, or SEO rules.
Use it only for transparent branding on a domain you own.

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Iframe masking page — crude "cloaking"
which can break login/forms/mobile/SEO
and may be blocked by the destination site.

A basic iframe masking page would be placed in public_html/index.html: