The link to create the integration key is missing in my settings page.
I'm using a trial account.
"API integration is only available with Enterprise and Developer"
Does that mean that we can't fully test your product with a trial account ?
Adobe Sign trial account has all the feature of Enterprise account except the Integration.
You can send a request for the free Developer edition of the Adobe Sign here Create Developer Account, APIs for custom applications | Adobe Sign
Please make sure that you do not use the same email address under which already have the trial account.
Use a different email address for the demo.
Also, would like to inform you that the settings are not enabled by default to access the Integration Key in the free Developer account.
I would recommend you to contact Pre-Sales team to get the settings enabled. They will be able to provide you pre-sales technical assistance.
Please refer this link Subscription plans for eSign services | Adobe Document Cloud to get in contact with them.
I hope this information will help.
Let us know if you need any further assistance.