OK, you will need to program the radio as PMR rather than LMR (Model -> PMR), and then use the 5-tone capabilities of the radio using DTMF format. So, under 5-Tone -> RX Code CH, set the first line to whatever string of digits you want to set off the horn, and turn "DEC EXO" for that line ON.
Under Common -> Common, turn RX EXO on and set RX EXO time to however long you want the radio to drive the horn output.
Then, under each individual channel, you will need to set, under "5-Tone Signalling," "Form" to DTMF from the default CCIR and make sure that the appropriate "RX C. No" is checkmarked (depending on what line you put your DTMF string in, under the RX Code CH menu from earlier.
Now, when that string is decoded, pin 10 on the DB25 should go low. The DB25 pinout document says this about that: "10 / HORNO / Output for horn switching signal (5mA max). OFF = H (When ON, input current must be lower than 50mA)."
Hope that helps!