There are two ways to approach this. You can use either one to get the same result.
1) In the MO8, set the Master Mode program so that the Zone addressing (transmitting to) the MODX does NOT send the Sustain message.
2) In the MODX, set the receiving Part so that it does recognize the Sustain message.

We assume you are using the MO8 Master Mode to address both the MO8 and the MODX simultaneously. One Zone to control the MO8, a second Zone to control the MODX.

Edit the MO8 Master program
In the MO8 recall the Master program
Press [EDIT]
Press the button [1]-[4] for Zone set to transmit to the MODX
Press [F3] TX SW
Scroll down to “SUS”
Set SUS = Off
Press [STORE] to update your Master program.

The same result can be accomplished in the slave device by instructing it to ignore the Sustain message on a Part-by-Part.

Edit the MODX Performance Part
In the MODX recall the Performance
Press [EDIT]
Select the Part containing the sound you want edit
Touch “Mod/Control” > “Receive Sw”
Set Sustain = Off
Press [STORE] to update your Performance.

BOTH of the above accomplish the same result. Use whichever one is more convenient.

Thank you Phil. I modified the setting on the MO8 master program - turning off the FS setting on that particular zone (SUS wasn't in the list of available values on the TX SW menu). I greatly appreciate your help as always. Thank you.