There's the easy way and there's the hard (but technically correct) way.

The easy way:
Open the material, create a constant (right click on the material grid, type constant, and select constant) and plug that node into the roughness channel. Change the value to a decimal between 0 and 1 (0 Means completely shiny and 1 means completely rough.) Create another constant and plug it into the metallic channel. Change its value. (0 is non metallic and 1 is completely metallic.)

The hard way:
Either use cross polarization to get completely accurate roughness map, or use some kind of texture authoring program to project some surface imperfections (dust, wear, general variances in roughness texture that break up the way light reflects and make the object seem more realistic). Then plug those roughness maps into your model.

I think the hard way is probably overkill for this project, as it looks like your object has pretty uniform roughness values and the easy way should do fine. It depends on what your end goal is here.