Hello, WixZ,
On the Translucency tab of your material, there is a “Lighting Mode” selector. Set it to “Surface TranslucencyVolume” and you will see the Metallic, Specular and Roughness pins enabled.
Please note that specularity on a translucent material has a lot of limitations and will probably behave different to what you expect, certainly different than what you see on an opaque material. Doing a realistic glass shader, for example, is not as straightforward as it sounds.
All the best.