Is it possible to add a custom string value for 1st Sunday each new month?

Example: 

FREQ=MONTHLY;BYDAY=SU;WEEKINMONTH=1*

*WEEKINMONTH: Not sure if this function is available. But I want to select which Sunday in a specific month that should be active or not. 

OR 

Is there any other smart solution for this? 

Try this: 'FREQ=MONTHLY;BYDAY=SU;BYMONTHDAY=1,2,3,4,5,6,7'

There are a lot of examples here: http://www.kanzaki.com/docs/ical/rrule.html

Thanks @Miles Rincker ! I have now added the rule and will test it later today if it works. Thanks again!