Please check the following link
How To: Create an ArcGIS Online Organization and access ArcGIS Pro after purchasing a Personal Use s...
Steps to activate ArcGIS Online:
1. Login to My Esri (Using the link with Fulfillment ID) > click Activate Subscription.
2. A screen will appear to select the account activation process option, select the first option, 'By creating a new account'.
If you have previously had a public account and want to convert it to an Organizational Account, please select the second option, 'By converting an existing ArcGIS Public Account'.
3. Fill in the requested information.
Steps to activate ArcGIS Pro+Extensions license(s):
4. Once the organization is created in ArcGIS Online, assign the ArcGIS Pro license to the ArcGIS Online user (Organization > Licenses).
5. Sign-in to ArcGIS Pro using those credentials.
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