Well annoyingly enough you don't want it to be marked as "Content" and you cant compile it, so I placed the following line into my .csproj file quite near the bottom. I have included the targets above so you can see where I placed it in bold.
<PropertyGroup>
<PreBuildEvent>
</PreBuildEvent>
<PostBuildEvent>
</PostBuildEvent>
</PropertyGroup>
<Target Name="AfterBuild">
<CreateItem Include="$(ProjectDir)ThirdPartyLibraries\PowerUp.Web.UI.WebTree.lic">
<Output TaskParameter="Include" ItemName="LicenceFile" />
</CreateItem>
<Copy SourceFiles="@(LicenceFile)" DestinationFolder="$(OutputPath)" ContinueOnError="false" />
</Target>
I created a target on the fly using the CreateItem step which also allows me to extend it later if I want to place more items inside it, then just referenced that item and copied it