Go to Settings > Integrations and Keys, click New key, and search for Todoist. Click Continue, specify a key name and click Save.
In the Integrations list, where your key is now included, click Authenticate..
Provide the header value for 'Authorization' in Todoist.
Type the word 'Bearer' (without the quotes and followed by a space) and then, after the space, the API token obtained from Todoist below.
Now that you've configured your integration, add one or more of the integration's actions to an automation, test the automation, and then add the automation to an incident type.