http://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/9/Dynamic-content/Dynamic-content/

The Dynamic Content concept was introduced in Episerver CMS before blocks became available. Blocks and dynamic content are somewhat overlapping concepts and we generally recommend blocks where possible. Since Episerver CMS 7.5, it is possible to drag and drop a block into a rich text area which makes it possible to use the block as a "dynamic content" in the text.

If you still want to use dynamic content instead of blocks, you can try the following code:

[DynamicContentPlugIn(DisplayName = "My dynamic content")]
public class MyDynamicContent : DynamicContentBase, IDynamicContentView
{
    public PropertyString MyProperty { get; private set; }

    public MyDynamicContent()
    {
        MyProperty = new PropertyString();
        Properties = new PropertyDataCollection { { "MyProperty", MyProperty } };
    }

    public void Render(TextWriter writer)
    {
        if (MyProperty != null && MyProperty.Value != null)
        {
            writer.Write("<p>" + MyProperty.Value + "</p>");
        }
    }
}

As Dejan says, blocks can also be used in tinymce editor and is the new way of doing this. Dynamic content is a dying concept so if possible, stick to blocks. Much easier and more powerful. What's not to like :)

I was able to make my dynamic content with Dejan's example. I missed using the Properties property. And by overriding the "RenderAsBlockElement" property i was able to make it inline.

The reason iam doing this with dynamic content instead of blocks is because the documentation recommends it.

The main limitation with blocks in rich text is that this is always treated as a block level element. Dynamic content can be treated as either an inline or block element. So if you need to insert inline content, for example, the latest company results or a phone number, dynamic content is still the preferred way to do this.

Thanks for you help guys! :)