It is not clear to me what you are asking. If you have a uBITX then the method that Bob describes is the easiest. If your question is how to do that, then here is the way that it is done in the v3 and later, displaying the v6 schematic:
The way that works is that the first mixer (T1 and T2) is being unbalanced by a 5 volt signal from the Nano on the CW-KEY line. This forward biases 2 of the diodes in the mixer and allows the LO signal that has been changed to the frequency that is to be transmitted to be applied to the amplifier stages.
It should be noted that the power output will be the full power for that band.
The other way would be to use FT8 software or similar to generate an audio tone that can be adjusted in level for the power needed to activate the tuner.
A third option would be tu use CW-TONE or generate a tone on the unused A7 pin to feed the mic input through a variable resistor. This would take modifiying the Nano code.
Of the three, I use either the CW or Audio tone from FT8 software.