Hey Kay,
There are a few ways to handle your question. The first is to look at Tom’s response here: Redirect to Login or Application After Activation - #5 by dutty
If you were to use Tom’s proposed flow, your app would be in-charge of displaying screens to complete the registration of the user via Okta’s APIs after you received back the activationToken (his flow assumes user is created with a password).
If you don’t want to handle the setting of the password on your screens you can setup your Okta activation emails to redirect after activation. The redirect url must be a Okta relative url:
${activationLink}?fromURI=/home/oidc_client/0oaa0my1wynLvWVDS0h7/aln5z7uhkbM6y7bMy0g7
The drawback to this is that ALL activation emails from your Okta org will be sent to this url. If you have any other types of users in your org setting the password in your application probably makes the most sense for you.