c# startup object not showing code example

Example: startup object visual studio c# not showing up

/*If the class that you want is not showing up in the drop down,
you can double click on the project in the Solution Explorer and 
add the class name after the project name.
(shown in code as "myproject" and "myclass")*/

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

<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp3.1</TargetFramework>
<StartupObject>myproject.myclass</StartupObject>
</PropertyGroup>

</Project>