That's a tougher question.
The only way I'd know how to do that is to leave the values in a string, split it, and then use mvindex to index the 12th value in the split.
| eval message = "this,is,the,fourth,fifth,sixth,value"
| eval new_mvfield = split(message,",")
| eval FOURTH = mvindex(new_mvfield,3)
| eval FIFTH = mvindex(new_mvfield,4)
| eval LAST = mvindex(new_mvfield,-1)
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