How do you calculate the electronegativity of a molecule?

Electronegativity is defined as the ability of an atom in an molecule to polarize electron density towards itself. There are various scales, and unsurprisingly, those elements towards the right of the Periodic Table tend to have high electronegativities: you know the drill, electronegativity increases across a Period (left to right), but decreases down a Group.

On the other hand, molecules may have formal negative or positive charges (of course these are ions), but we cannot speak of their electronegativity in that they are MOLECULAR not atomic entities.

If you meant how to determine the dipole moment, then, for example, if you have #"HF"#, then the dipole moment is along the internuclear axis. It happens to be #"1.86 D"#.

If you take the bond distance in angstroms (#"Å"#) and you multiply by the electronegativity difference, you can approximate the dipole moment.

#mu = "0.92 Å" * 1.9 = "1.748 D"#, which is somewhat close to #"1.86 D"#. The actual value is more closely approximated via computational chemistry, and you don't need to do that.

If you meant atoms, then read this, which delineates how electronegativities were determined by Linus Pauling.