Creating the ACF Relationship Field
Relationship Fields are a powerfull functionality within Advanced Custom Fields. Thanks to Rexy, you can use that power and combine it with the Post Widget of Elementor.
Search for Custom Fields in your menu panel
Click Add New
Give your Field Group a relevant name.
Click on the + Add new button and give your ACF Field a name.
Scroll down and select Relationship as type of your ACF Field.
You will see some fields appear that are related to this funcitonality. Choose the right filter for you field and choose where you want your Relationship Field to appear.