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There are 2 best solutions below

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Shahzad Qureshi On BEST ANSWER
  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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Sandar Min Aye On

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