I am monitoring the user's location every 15 minutes and I just want the application to continue sending the location even if the user closes the application in the taskbar.
I tried this sample but it's in Xamarin.Android https://docs.microsoft.com/en-us/xamarin/android/app-fundamentals/services/foreground-services i have to create a dependencyservice but i don't know how.
You might want to have a look at Shiny by Allan Ritchie. It's currently in beta but I would still suggest using it, as it will save you a lot of trouble writing this code yourself. Here's a blog post by Allan, explaining what you can do with Shiny in terms of background tasks - I think Scheduled Jobs are the thing you're looking for.