Here’s how to use the 3ds Max object ids for a simple Object ID AOV.

And here’s the material (aka shader tree) that writes the AOV. Use a text editor to save this as Arnold Scene Source (.ass) file, and then import it into 3ds Max (like I did in the video).

An ASS file is a plain-text file.

### exported: Thu May 21 07:53:19 2020
### from:     Arnold 6.0.3.0 [991b08e9] windows icc-17.0.2 oiio-2.2.1 osl-1.11.0 vdb-4.0.0 clm-1.1.1.118 rlm-12.4.2 optix-6.7.0 2020/04/17 09:11:12
### host app: MAXtoA 4.0.4.36 (2021) 3ds Max 23.0.915.2021 

aov_write_rgb
{
 name /Write_Object_ID_AOV
 aov_input /gBufID_Switch
 aov_name "object_id"
 declare nodeName constant STRING
 nodeName "Material #43"
}

switch_rgba
{
 name /gBufID_Switch
 index /gBufID
 input1 1 0 0 1
 input2 0 1 0 1
 input3 0 0 1 1
}

user_data_int
{
 name /gBufID
 attribute "gBufID"
}