Modify (linked) Image width to 100%

I would like to automatically set the image width to 100% when a new image is added. More particularly:
A site owner (me) is in the dashboard of a page.
While modifying I add a new image using the “insert/edit image” button above the editor (Note: I am NOT using “add media”), thus specifying the location of the image via URL within the “insert/edit image” dialog that pops up.
I do not want to modify the image dimensions but just having the image width automatically set to 100% when clicking the “OK” button.
I tried various hooks and searched in the wordpress core but could not find any hook or function that relates to my issue.

Thank you in advance!

Ps.: CSS is no option for me.