If Formula for several columns with text and blanks - Help

I cannot for the life of me figure out what IF (assuming that is what I need) formula to use for what I am trying to accomplish, if anyone can help, I would be so grateful!

I need to:
If in Column E a cell contains the word "Highland" then
in Column S if the corresponding cell in that row is blank,
I want it to enter text "Use Alt Form"
However, I also need to be able to overwrite the formula and enter text/date at a later time

Example:
E3 says contains Highland & S3 is blank, I need it to auto-populate to contain Use Alt Form
and have that continue through my entire worksheet for those columns

If E4 says San Marcos, I want S4 to stay blank

Hope this makes sense...

Thanks!