Transforming web.config before publishing to server with WebDeploy (TransformXml)

I have a WebAPI / EntityFramework project that I am working on and I publish updates to the server quite often (using WebDeploy).

My setup uses custom defaultConnectionFactory in order to create connection string based on certain criteria.

Every time I push an update to sever, an new connection string is added to my web.config file:
name: MyProject.Lib.Data.DataContext
value: MyProject.Lib.Data.DataContext_ConnectionString

I found a solution (http://stackoverflow.com/a/3622821/573208) that looks like exactly what I am looking for but I could not make it work.

I added this line at the end of my project file

<UsingTask TaskName="TransformXml" AssemblyFile="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v14.0\Web\Microsoft.Web.Publishing.Tasks.dll" />

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right before </Project> closing tag. But could not go any more further.

The article states that I need to

place <TransformXml Source="web.config" Transform="web.release.config" Destination="$(DeployPath)\web.config" /> in the right item group

and then

make sure that target gets fired prior to the call to Package

That's where I got confused:
- Where should I place the <TransformXml ... /> section and how to I find out which is the "the right ItemGroup"
- How do I "make sure that target gets fired prior to the call to Package"

Any help would be much appreciated