While the OP asks concerning a Futaba radio, this is an Off-Topic answer. OTOH some may read it and learn something that I have not seen addressed in some time.

A JR Buddy-box system (my older one-haven't used a newer one for training) powers up when used as a buddy box (BB). That meant having a charged battery in it and for certain the antenna in, or having the freq. pin for both the master and the slave BB.

TIP: if the JR BB you are using is showing power on when activated with the switch/button, for BB use only, one can configure the BB to accept, via a homemade connector a simple common square rectangle 9.0 volt battery. This is plenty for the BB to work and it lasts a long time. One can remove the antenna and/or remove the crystal and obtain good use.

I use an old Futaba 4-channel Tmttr for a BB with either Futaba or Hitec. If you are using a Hitec transmitter as the primary, then watch out for the BB to transmit. BTDT I suggest either remove the BB antenna or the crystal.

Using 72 MHz for training I like to have BB things dead so as to make the student pull out the antenna, then shove it back in before taxi out. It trains the newbie to do so if he is using other than 2.4.

Pretty soon no one will even know what an antenna is!!