Hi @M_Santi. First, how was that custom iProperty created? Manually, though the main iProperties dialog, on the Custom tab? Or was it created automatically by marking a Parameter within the Parameters dialog to Export (Checkmark in the 'Export Parameter' column for that parameter)?
If it was made manually, then you will have to open the Event Triggers dialog on the Manage tab, iLogic panel. Then make sure the tab named 'This Document' is the active one. Then drag this iLogic rule from the list on the left, over under the 'iProperty Change' event on the right. Then click OK.
If it was created automatically from an exported parameter, and this iLogic rule in an 'internal' one (saved within the Inventor document, not external), then we could likely just include the name of the unquoted name of that parameter somewhere within that iLogic rule. To do that, make a new, empty line at the top/start of that iLogic rule, then create a 'dummy variable' like "oTrigger = MyParamName" (without the quotes). The name of that parameter will turn blue (by default), signaling that it is recognized as representing a 'local' parameter (within that same document). Then, whenever the value of that parameter changes, it will cause this rule to run automatically.
Wesley Crihfield

(Not an Autodesk Employee)