You can’t directly set TargetFramework based on the CPU platform in a single project file.
However, you can achieve this by using multiple configurations in your project.

Create separate configurations for x86 and x64 in your .csproj file and specify the corresponding TargetFramework for each configuration.

For example:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <RootNamespace>YourNamespace</RootNamespace>
    <AssemblyName>YourAssemblyName</AssemblyName>
    <TargetFrameworks>net45;net8.0</TargetFrameworks>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x86'">
    <TargetFramework>net45</TargetFramework>
    <PlatformTarget>x86</PlatformTarget>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x86'">
    <TargetFramework>net45</TargetFramework>
    <PlatformTarget>x86</PlatformTarget>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
    <TargetFramework>net8.0</TargetFramework>
    <PlatformTarget>x64</PlatformTarget>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
    <TargetFramework>net8.0</TargetFramework>
    <PlatformTarget>x64</PlatformTarget>
  </PropertyGroup>

</Project>

• When building your project, select the desired configuration and
  platform (e.g., Debug/x86 or Release/x64) in Visual Studio.
• Visual Studio will then use the settings specified in the corresponding
  PropertyGroup.

Rather than thinking of setting the target framework based on the CPU platform, maybe reverse it and set the CPU platform based on the target framework; consider:

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFrameworks>net48;net8.0</TargetFrameworks>
        <LangVersion>latest</LangVersion>
        <PlatformTarget>x64</PlatformTarget>
        <PlatformTarget Condition="'$(TargetFramework)'=='net48'">x86</PlatformTarget>
    </PropertyGroup>
</Project>

with

using System;
Console.WriteLine(Environment.Is64BitProcess);
Console.WriteLine(IntPtr.Size);

which we can test:

➜ dotnet run -f net48
False
4
➜ dotnet run -f net8.0
True
8

(obviously adjust to your chosen targets; I’ve used net8.0 and net48 for my convenience)