I found this extension (Default Selection) which works quite well. It took me a while to get it to work (I'm a newbie too), but I finally got there. Hope this helps.
Firstly, regardless of which "Initial Selection Mode" you chose, make sure you clear all selections, save and close the sheet, then reopen the App and sheet to see the latest changes.
Secondly if you are using a time field like Year-Month start with a simple text value to make sure that works.
Initial Selection: ='2018-Oct'
Once you are happy it is working, then play with the expression and make sure it evaluates to a string that matches your field selector; not a date value.
My formula is this...
=Year(AddMonths(Now(),-1)) & '-' & Month(AddMonths(Now(),-1))
Finally, you may want to Hide the extension object (un-tick Visible)