In order to write bits within a word your device must support function 22 (masked bit write).
A work around could be to set OutputType to WriteValue and set ValueToWrite to the bit value (1,2,4,8,16, etc). With this method your PLC will need to turn off the bit and any button will set its bit and clear all others.
You could also write the values using code by reading the value, setting the bit, then writing the value back. There is the risk that a bit could change value in between the read and write making the write put that bit back to the previous value.