You need to create an acf/prepare_field filter and get the values from the options.
This documentation covers the basics https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/
But you should use the prepare field filter https://www.advancedcustomfields.com/resources/acf-prepare_field/ rather than the load field filter. acf/prepare_field was introduced after the other document was written
