Wanting a column to fill with certain values based on if data is placed into other cells

Hello!

First of all, thank you for taking the time to read this and help.

So I am looking to create usernames and passwords for bulk uploads.

Example,

If cells A3, A4, A5, and so on are their first names, and cell B3, B4, B5 and so on are their last names. How can I set column E to automatically populate their first and last name based on if data is present. So E3 should be equal to A3 and B3. I know the formula to get the text to add together =LOWER(A3)&"."&LOWER(B3), but how do I make the column keep going with that formula? That way if I get a list with 400 names I don't have to type in the formula 400 times.

I am also looking to create the default password, so something like if there is text in A3 and B3 use this text in F3.

Thank you so much for your time.

Welcome to the board.

Try

=IF(A3="","",LOWER(A3&"."&B3))