If you are just looking to populate "Assign To" with the identify of the person who is inserting the row, the easiest way to do that is to use the USEREMAIL() function. Place this in the Initial Value property of the "Assign To" column and it will automatically populate the field with the email address of the logged in user.
Otherwise, if you need something different, you need to tell us, in words, how it is supposed to be populated for your specific app. We also need to know what data table you have to support your need. Then we can help with an expression.
LinkedIn
Twitter
