The only way to accomplish what you want to do with ACF is to create a radio field that lets the user select the type of link, for example internal or external and then to have a page link or URL field that are both conditional base on the radio field.
Then where you’re getting values on the front end you check the radio field and then get and display the value for the link from the right field.
The other choice is to give them a text field that they can enter whatever they want into it. but the first option also allows you to set up if the link should be opened in a new tab.
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