Javascript OnBlur - Is this possible?

Hello,
I have a form with a textbox (StudentID). If the user enters their StudentID,  I am trying to populate a textbox (txtFirstName) with the (firstname) from a stored procdure that has the same ID. I have added an OnBlur function to the txtStudentID but I am not sure how to set my variable in my Javascript to the firstname field in the stored procedure. I am currently getting an error that the firstname is undefined (which makes sense). Here is my code:


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