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?
You might want to take a look at the acf/load_value filter https://www.advancedcustomfields.com/resources/acf-load_value/
