BDD script for performance of text showing up on summary pane as I type

I want to write a BDD for performance requirements but am not too sure how to separate out the UI details from the requirement itself in this case. I have got a summary plane in my application where text that I enter in my main form as I type appears in the summary plane. The requirement I wrote is below:

Given as a User
When I fill in "Bob" as "Customer Name"
Then "Bob" appears as "Customer Name" in the summary plane

I got two questions. Is this advisable and how would I add details that the name "Bob" should appear in the summary plane as I type and not appear later ?