--key=<licensekey>
You'll find your ACF PRO license key at https://www.advancedcustomfields.com/my-account/
wp acfpro activate --key=XXXXXX
Most of the code is from https://anchor.host/preloading-advanced-custom-fields-pro-license-key/
wp-cli-acfpro-activate is copyright 2019 Per Soderlind
wp-cli-acfpro-activate is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version.
wp-cli-acfpro-activate is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with the Extension. If not, see http://www.gnu.org/licenses/.