In order to access Canva, you would need to raise a ticket for the Service Desk requesting access to the application, which would be recharged to the department / project at a cost of £15 per user, per month.
Note: Your line manager or project manager along with Internal Communications will need to approve your access to the app. The screenshots below may be slightly different for you, based on the level of access that you have within the application.
Signing in to Canva
After navigating to the URL, click the Log in button.

You are presented with a number of sign in options, please select the bottom option Sign in with your work email.
Add in your ASI / project email address and click Continue.
If you already have an active Microsoft session in the web browser, you should automatically be signed in, if not, you will be asked to sign in to the Microsoft service.
You will need to provide a password (to confirm it is you accessing the account) and then confirm with a Multi-Factor Authentication response (Microsoft Authenticator application).
Once you have confirmed it is you accessing your account, access will be granted.
Use of Teams in Canva
By default, you will be added to the Adam Smith International team in Canva.
If you have used your account to access other Teams before now, you should be able to access them by clicking on your initials in the bottom-left hand corner and then selecting Teams.