Addendum to the NA64 Proposal: Search for the $A'\to invisible$ and $X\to e^+e^-$ decays in 2021

The experiment NA64 is aimed at a direct search for sub-GeV vector mediator $A'$ of Dark Matter production in invisible $A'$ decay mode. Another goal is to search for of a new light $X$ boson, which could explain a recently observed excess of $\ee$ events from excited $^8$Be transitions. The NA64 Collaboration requests to carry out further both searches with the H4 electron beam in the year 2021 and beyond.