Solved
How to change the default action in conflict prompt
Is there a way to change the default action in the conflict prompt from "Check out now" to "Make Read-only" for example?
Not, via this dialog. You can default to the other values. See https://www.ptc.com/en/support/article/cs44835
For 'Read Only', consider setting the Windchill preferences that automatically set everything added to your workspace as read only.
https://www.ptc.com/en/support/article/cs76228
https://www.ptc.com/en/support/article/cs334870
https://support.ptc.com/help/wnc/r12.0.2.0/en/#page/Windchill_Help_Center%2FCADxFileMenuObjAdd.html