I am not sure about the normalisations and your contraction is wrong since the charge should be a scalar quantity but you can play with the indices using the fact that

$$ F_{ab} F_{mn} dx^a \wedge dx^b \wedge dx^m \wedge dx^n = F_{ab} F_{mn} \, \epsilon_{efgh} \epsilon^{pqrs}dx^edx^fdx^gdx^g \delta_p^a \delta_{q}^b\delta_{r}^m \delta_s^n$$

possibly times some normalisations that you can fix later. I trust this is correct but I would advise to check it out in books like Freedman's and Van Proyen's Supergravity or similar. After you do all the horrifying horrible contractions you should probably get what you want.