I have examples of setting fields using jQuery, but I have not done a relationship field. I did do this for a date field recently for another post. This had to do with interacting with the jQuery datepicker itself as apposed to interacting directly with the ACF field. I’m guessing that a relationship field would be similar due to the fact that you’re dealing with a select2 field and would probably need to interact directly with setting the value by calling some select2 function.
Anyway, the example for the datepicker field is here https://support.advancedcustomfields.com/forums/topic/autocomplete-end-date-when-start-date-is-selected-in-jquery-date-picker/
and my other examples are here, they might help. https://github.com/Hube2/acf-dynamic-ajax-select-example
