Hi Ronan,
I will make some assumptions about what you are trying to do. A screenshot would be helpful to understand.
Have a look at this item about creating a multi-select box that has filtering capability. To do this essentially, just use a listbox, not a combo box, give it a name, then drag on the select2 block, and specify your named listbox's id in the select2 block.
https://www.outsystems.com/forums/discussion/32689/select2-multiple-selection/
Is that what you were looking for?
Kind regards,
Stuart