For the OGOship integration you need to generate API key from Squarespace.
In your webstore's page go to Settings -> Advanced -> Developer API Keys.
In the "Developer API Keys" page click "GENERATE KEY".
You can generate API keys from here. It is strongly advised to grant all the permissions for the API key in order for the integration to work properly. Name the key and select each permission and option "Read and Write" for them. Then click "Generate key".
Once the API key is generated you can copy the key. This key will be used for the integration.
Go to myOGO "Edit merchant" page and select "Integrations". Click on "+ New integration" and select Squarespace from the list.
Paste your Squarespace API key to the API key section.
Remember to save your changes. You can test your integration by pressing "Test" button.
Once you have integrated Squarespace you can synchronize your products with OGOship. Instructions are on product sync page.
Read also about setting up shipping methods.