You can get the material from the landscape since it is just an actor. The landscape creates sections as a component of the landscape. You can get these components and create instances for it.
Now, if you want all the sections to have the same parameters you are better of creating a Material Parameter Collection, since every section will have it’s own material assigned optimized for that specific section (i.e. if 1 section does not have grass it will compile a shader for that section without grass in it to save performance on texture look ups and blending). This means every time you update a parameter you would have to do it for all the different material instances instead of you changing it in the material parameter collection.