I'm trying to create an anonymous type based on string parameters for example,
var propertyName = "Firstname";
var properyValue = "Fred";
I would like to be able to get;
var myObj = new { FirstName = "Fred" };
But also having the opportunity of using the same function and having;
var propertyName = "Surname";
var properyValue = "Smith";
var myObj = new { Surname = "Smith" };
Is this actually possible?
Anonymous types are defined at compile-time. They're normal C# types, just without a name [1]. As such, you cannot modify their structure based on runtime information.
It doesn't seem like you want anonymous types at all, really. ExpandoObject sounds more like what you want. Anonymous types shouldn't leak from their scope - and if you follow that, you don't ever need a way to construct them dynamically like you're trying to do.
Additionally, be careful when you expose an anonymous type. There's a reason why they aren't allowed to be used as method arguments and return types - that isn't what they're designed for. It seems you're circumventing this limitation by using dynamic, but that has many problems; it can stop working with any release of .NET (including hotfixes), and with current .NET, it will cause you accessibility errors if you leak the anonymous typed to where it shouldn't be (e.g. an internal anonymous type used in a different assembly).
[1] Of course, they actually do have a name, since the CLR requires types to have a name. But the name is autogenerated and somewhat hidden, and not something you should ever take into account.
Dictionary<string, string>and live happy.ExpandoObject, but why do you want to do that? It might be possible that you've got a XY Problem