Hi Ido,
See if this works for you:
CrystalDecisions.Windows.Forms.ParameterPromptControl
We don't have any sample unfortunately... You'll have to override it.
Other option is to create your own Parameter prompt and use yours.
For examples of replacing our Viewers buttons with your own search this KBA with an example of replacing our export button:
https://i7p.wdf.sap.corp/sap/support/notes/1765153
Don
PS - this is not supported. API's are not public, if they are not in the Help file they are not officially supported.
Add your request to Idea Place to add a click Event handler for the parameter UI.