I want to add an external solar panel and plug it into the Anderson plug, will this work?

If your battery management system has an inbuilt solar regulator, unregulated solar panels should connect directly into the unit (J35 or BP35) solar input terminals. There is no need to install an external regulator between the solar panel and the J35.

If you want to add an additional regulated solar panel or a blanket panel, it needs to go directly to the battery, and would require a BC300 fitted so that your battery monitor (JHub, Odyssey, Trek, RVView, JControl) reads correctly with the coulomb counting.

If your J35 is not solar compatible (J35A and some earlier versions of the J35B, BatteryPlus35-PM), then you may install your solar panel direct to battery through an external regulator. So that your battery monitor sees the solar charging current at the battery, you will need to also connect the negative through the BC300 shunt.