You can map XML namespaces to assemblies using a series of tokens within an xmlns prefix declaration, similar to how the standard WPF and XAML-intrinsics XAML namespaces are mapped to prefixes.
The syntax takes the following possible named tokens and following values:
clr-namespace: The CLR namespace declared within the assembly that contains the public types to expose as elements.
assembly= The assembly that contains some or all of the referenced CLR namespace. This value is typically just the name of the assembly, not the path, and does not include the extension (such as .dll or .exe). The path to that assembly must be established as a project reference in the project file that contains the XAML you are trying to map. In order to incorporate versioning and strong-name signing, the assembly value can be a string as defined by AssemblyName, rather than the simple string name.
Note that the character separating the clr-namespace token from its value is a colon (:) whereas the character separating the assembly token from its value is an equals sign (=). The character to use between these two tokens is a semicolon. Also, do not include any whitespace anywhere in the declaration.
Taken from here.