Use a form rule to change the value of the related field to the max record id since you already know it
CAN YOU TALK MORE ABOUT THIS PLEASE, I WOULD LOVE TO HEAR YOUR ANWSER.
Golden - to clarify something from your original comment and Michaels suggestion - when you say the 'Max Record ID#' of the parent table - do you actually have a field that represents the max ID - or is it the case that you have a dropdown on your form, and in that you are always scrolling down because you always want to pick out the max record ID during your entry?
I'm assuming the latter based on reading your full post. If thats correct - you can do something quick/easy and just reverse the order of your dropdown and sort highest to lowest on RID so that the max RID is the first option - not the last. The other option is to actually use your architecture to summarize a value into your parent or some other table of what the Max Record ID# is and then create a form rule per Michaels comment.
IF you already have a field that represents the max record ID# - then your form rule would just be something like:
When Related Parent is blank change value of Related Parent to Max Record ID#
