How to have Sprint Open Condition

[Gor Greyan]

Hello dear colleagues.

I have one question regarding to Sprints condition.

I have the following situation.
I need when on the Story added a Sprint for the first time, Original Estimate is being copied to the one custom field and will not be edited further. 
I have automation, which trigger is Sprint change, but I need to check it, when only first time sprint is added to the task, how to do it?