It really depends on what you are trying to do, what the ultimate goal is.
Do you want this to be the default value and have it displayed on the site if nothing is entered into your field? or do you want to override whatever is in this field with the default value? Something else?
sorry for this long interruption. The ultimate goal is:
If i save the post/page/customposttype:
– If nothing is entered in that field, a function insert the name of the post/page/customposttype in to the acf-field.
I’m sure, it’s easy, but didn’t found the right solution.