To enter the pro code simply do following

The example shows how it looks like on an android device - logically, this applies to an iphone and windows computer as well.

• open the settings of the watchface inside your mobile phone or computer
• scroll down to the very last setting
• enter the unlock code inside the Pro Code field

Depending on your device this field may look like its clickable or not - if it looks like following simply click the title of the pro code field to open the dialog where you can enter the code.

Sadly this user interface is totally under garmins control and I have no influence on it.