This post tells you how to create a material parameter in Unreal Engine 4
How To: Create a material parameter in Unreal Engine 4
While editing a material, hold ‘1’ (or search for “constant”) on your keyboard and left click on an empty space. A new constant will be created.
Now right click on it and select ‘Convert to parameter’ option. Give the parameter a meaningful name.
Now you will be able to quickly edit it from the material instance details panel.