[.NET Core, Jenkins] Help with sensitive data

I would like to optimize the deployment process with Octopus tools. My technology stack is a .Net Core application (WebAPI + additional .dll type projects). I have successfully gone through the tutorial and created a deployment process based on git, jenkins, Octopus Deploy -> IIS.
However, I have a big problem with the data that is retrieved when the application is turned on, this is private(sensitive) data such as connection strings, encryption keys etc. Is it possible to keep them in secure place in octopus deploy server and then pull them in startup class depending on supported Dev/Test environment? I looked through the tutorials, but only came across swapping data in the appsettings/webconfig file.
Many thanks for any hints.

Apologies for the late reply.

In your particular use case, and assuming you are using .NET Core, have you had a chance to look into environment variables? https://library.octopus.com/step-templates/3586C580-7FC8-4FAB-A783-3641D91F7746/actiontemplate-asp.net-core-set-environment-variables-via-iis-config

You’d store the sensitive values in Octopus as sensitive variables (Sensitive variables - Octopus Deploy) which you’d then use as environment variables for your site.