I have a C# Asp.net web project made in Visual Studio. The project runs on a certain port (57243) and I made other programs that were testing the web service etc to use "localhost:57243".
Recently I tried running the project in Jetbrains' Rider IDE because of whatever reasons I made up at the time. The only issue I am having now is that the web service runs on port 5001 and I cannot find any property to change the base url or the host port to make it work.
TLDR, I am looking for this option inside the Jetbrains' Rider IDE:

This can be done inside the Rider IDE if you like.
If you edit the run/build configuration you are using when you hit F5 and then edit the environment variables you can add the ASPNETCORE_URLS environment variable which will run the app using the port specified as shown here.
Rider Run configuration environment variables
Steps to edit the setting
ASPNETCORE_URLS and the value to http://*:57243@Sasquatch's answer works for ASP.NET Core only.
For plain old ASP.NET, with IIS Express, we can go the project properties, Web section, and then
This will rewrite the generated applicationhost.config file (in .idea\config\ folder) with your selected configuration.
[*] If "Generate applicationhost.config" is unchecked, you should edit that file directly, like @WWietlol's answer suggests.
It turned out that the option in Visual Studio just changes the application.config in the .vs folder. A similar file in the .idea folder had the properties of the ports.
Changing it in that file fixed it.