You will need to complete all 5 locations of the Sunriver Adventure Lab Cache to obtain the coordinates for this Bonus Cache.
While completing each location of the Sunriver Adventure Lab Cache, you will be given a part of the final coordinates for this Bonus cache by clicking on "View Journal". Plus, after completion of all 5 locations of the lab cache, you can click on "View 5 Journal entries" to review the bonus information by each location.
Due to COVID restrictions this bonus is a little away off the beaten path. There are other caches in the area for you to find.
Be sure to bring a writing stick.
Have fun, stay safe and hope you enjoy this adventure.