When adding a Squarespace sales channel, you're met with a prompt for your Squarespace site's API key. Don't worry, it's super easy to set one up to use with Trunk.
Please note that you will need to at least be on Squarespace's Commerce Advanced plan in order for Trunk to access the necessary APIs. More information on this can be found here.
Head on over to your Squarespace site and on the left sidebar, click on Commerce, then Advanced, then Squarespace API Keys, then you should see a button to create a key.
Click the button and you'll be met with a window create your API key. Enter in a Key Name and then make sure both Inventory and Orders scopes are selected.
Then click Next to create your API key! Now all you need to do is copy the newly created API key as this is what you will be giving to Trunk. Also, this will be the last time you'll get access to this so you might want to save it somewhere!
Now head on over back to Trunk and when it asks for your Squarespace site's API key, simply paste in what you copied.