The challenge based system on FICS allows direct match requests and seeks. To post a seek to the seek graph, go to Board's Option button - Find a Game on menu. The left tab on top of Find a Game will switch it from seeking to seek graph. Green is regular chess with a human on Seek Graph and solid is rated and hollow is unrated. Blue is computers. To rematch the last opponent go to Actions button then on menu, rematch.
If one has opened one of one's recent games or loaded PGN the user is in Examine Mode on FICS and can't seek while examining. Go to Board's Action menu and select unexamine or type unexamine in Main View.
In seeking if a new player there is a default formula on FICS of Blitz or Standard games. Blitz is 3 minutes to under 15 and standard is 15 and up and this also excludes variants. If not seeking Blitz or Standard, have use formula off in seek form of Find a game Option, or type help formula on FICS to change it.
If it's not a rematch the only way to directly challenge a friend is to type match username time inc r/u. I.e. match rookers 5 0 r, for a rated game or match rookers 5 0 u, for an unrated.
To Withdraw ones seeks, challenges, and pool requests, the Find a Game option, Pool tab, withdraw challenges, can be used.