In .NET Core 3.0+ include the NuGet package
Microsoft.AspNetCore.Mvc.NewtonsoftJson and then replace
This is a pre-release NuGet package in .NET Core 3.0 but a full release package in .NET Core 3.1.
I came across this myself, but I’ve found that the same answer with some additional info is in this SO question and answer.
Edit: As a useful update: code with the call to
AddNewtonsoftJson() will compile and run even without installing the
Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package. If you do that, it runs with both converters installed, but defaulting to the
System.Text.Json converter which you presumably don’t want since you’re reading this answer. So you must remember to install the NuGet package for this to work properly (and remember to re-install it if you ever clear down and redo your NuGet dependencies).