Pull object Id from selected related polygon

[Baisuzhen]

In Field Maps I'm trying to use the OBJECTID field of a selected polygon for a unique ID. 

The final code would look like this:

Concatenate("PulledOBJECTID", "_", myDate);

This would populate within a field in the table that is related to the parent polygon. 

However, I cannot figure out the best way to pull the OBJECTID from the selected polygon to use in this code.

I've been using the FeatureSetByRelationshipName but I keep coming across errors so I was just wondering if anyone else has had to do this and has some advice?

Your help is much appreciated!

[gis_KIWI4]

@Baisuzhen - Are you able to give us more detail on the setup of the layers? 
Are you trying to calculate the attribute in the child record?
Also is the child record a table? 

[DougBrowning]

I would use something else as a key.  I like to calculate it based on some attributes.  If you ever export records, reload from backup, push to SDE, etc all your objectids will break and it will be impossible to fix.