Hi Marcel,
We used the below widget for one of the similar requirements
Mendix Marketplace - Bootstrap Multi Select widget
We have written one initalization microflow to fill the data for the checkbox and OCH microflows whenever the check box is being deselected. you can try this
Regards,
Ajay
If I understand the problem correctly, you can do this using the combo box widget...
The combobox widget can be configured to allow multi select with checkboxes if the association allows many selections.
You can then pre select the correct items using a microflow before the page opens
You just need to add the list of objects you want preselected to the association in your microflow logic