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There are 2 best solutions below

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  1. Open your IIS Manager
  2. Right click on the 3CX website and select "edit bindings"
  3. Click Add
  4. Enter the domain name you want to use and a port
  5. Click OK

This will make the website accept it, you have to make sure the DNS record is set up correctly.

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In Forward Lookup Zone of DNS Manager, I added new host (A) record and points to private/public IP Address.