Not mod, but in order to nominate relationships on the character field you have to put "Relationship:" before every ship, and AO3 will accept it. Example:

Relationship: John Childermass/John Uskglass | The Raven King

If the fandom has more versions with the same characters (for example a book and the movie inspired on it) you also should add the title in brackets at the end, example:

Relationship: John Childermass/John Uskglass | The Raven King (Jonathan Strange & Mr Norrell Novel)

I hope this helps.