Modbus RTU on iX - how to declare sub - indexes

[nxen]

Hello,

I'm trying to send values from a X2 Base panel to a Beijer BSD servo drive, via Modbus RTU.
To do this, I need to assign the servo indexes to the iX tags.
What I didn't find is to assign the sub-indexes. For example, how to declare the Index 0x3100 - Sub Index 02 as a tag?

I would appreciate for your advice.