As of writing (Unreal Engine v4.25), the default C++ standard of Unreal Engine is C++14.
On the other hand, note that EnTT requires C++17 to compile. To enable it, in the main module of the project there should be a <Game Name>.Build.cs file, the constructor of which must contain the following lines:

PCHUsage = PCHUsageMode.NoSharedPCHs;
PrivatePCHHeaderFile = "<PCH filename>.h";
CppStandard = CppStandardVersion.Cpp17;

Replace <PCH filename>.h with the name of the already existing PCH header file, if any.
In case the project doesn't already contain a file of this type, it's possible to create one with the following content:

#pragma once

#include "CoreMinimal.h"

Remember to remove any old PCHUsage = <...> line that was previously there. At this point, C++17 support should be in place.
Try to compile the project to ensure it works as expected before following further steps.

Note that updating a project to C++17 doesn't necessarily mean that the IDE in use will also start to recognize its syntax.
If the plan is to use C++17 in the project too, check the specific instructions for the IDE in use.

EnTT as a third party module

Once this point is reached, the Source directory should look like this:

Source
|  MyGame.Target.cs
|  MyGameEditor.Target.cs
|
+---MyGame
|  |  MyGame.Build.cs
|  |  MyGame.h (PCH Header file)
|
\---ThirdParty
   \---EnTT
      |   EnTT.Build.cs
      |
      \---entt (GitHub repository content inside)

To make this happen, create the folder ThirdParty under Source if it doesn't exist already. Then, add an EnTT folder under ThirdParty.
Within the latter, create a new file EnTT.Build.cs with the following content:

using System.IO;
using UnrealBuildTool;
 
public class EnTT: ModuleRules {
    public EnTT(ReadOnlyTargetRules Target) : base(Target) {
        Type = ModuleType.External;
        PublicIncludePaths.Add(Path.Combine(ModuleDirectory, "entt", "src", "entt"));
    }
}

The last line indicates that the actual files will be found in the directory EnTT/entt/src/entt.
Download the repository for EnTT and place it next to EnTT.Build.cs or update the path above accordingly.

Finally, open the <Game Name>.Build.cs file and add EnTT as a dependency at the end of the list:

PublicDependencyModuleNames.AddRange(new[] {
    "Core", "CoreUObject", "Engine", "InputCore", [...], "EnTT"
});

Note that some IDEs might require a restart to start recognizing the new module for code-highlighting features and such.

Include EnTT

In any source file of the project, add #include "entt.hpp" or any other path to the file from EnTT to use it.
Try to create a registry as entt::registry registry; to make sure everything compiles fine.

Table of Contents

Enable Cpp17

EnTT as a third party module

Include EnTT