Use the power of Twig to generate on the fly the Options of a Select in a Form.
In this scenario we will read the IDs of posts from an ACF Relationship field, which as you can see below it contain an array of numeric ID of selected posts.
array ( 0 => '1329', 1 => '932', 2 => '946', )
So use Dynamic Tag Twig on the Options field and we insert this snippet, you have only change the “acf_relationship” name with the ACF field name configured on your site.
Dubai|1329 London|932 Venice|946