Update an input textfield, on pageselect without save the whole page?

[August]

Hello? 
I'm not sure if it can be done and would be happy to read about from you:

Any approach to solving the problem would be really helpful here.

I use a template in which save locations as pages, fields here are storing title, address and so on to each location ..
Another template (lets call it B), I access the locations as page-reference-field (single select) and I would like to display the specification of a field (a simple string) from the location-template, which has Ryans Fieldtype “text field unique”. Using a hook on "text field unique in template B display the "unique" placeholder - so that these string cannot exist twice in the system.
By the way, it works, but only - ion page-save, first .

Do you see a way, that the text field with the required string is automatically filled out, if a location (Page Reference) has been selected, just before?

I also thought about doing this with another page selection reference field, but I don't see any way to update the required  text input placeholder …