To add your Airtable API key to your Softr app, you need to copy it from your Airtable account and add it in Settings => Integrations => Airtable. The process is demonstrated below.
Adding the Airtable API key on Softr
As you can see, when visiting the API key field after saving it, all the characters apart from the first and last three are masked, so that other people collaborating with you on the same Softr app don't have access to it.