First, make sure that the API is enabled for your TestRail installation. It can be enabled in the administration area in TestRail under Administration > Site Settings > API.

In Assembla click on the drop-down menu on to the right of your avatar at the top of the screen. Then, select Profile. Open the Single Sign-On menu from the left sidebar.
Click the "Connect account" button next to TestRail to proceed to connect your account.

You will then be prompted to provide your TestRail instance URL and click "Connect".

If you are not signed into the TestRail yet you will see the login page.

After signing in with your TestRail credentials, you will need to confirm Assembla authorization.

After accepting you you are going to be redirected back to Assembla where you will see your connected TestRail account listed under the Connected accounts section.
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Note: You can remove a connected account at any time by clicking Disable.

You have now connected your TestRail account to Assembla and can log in by simply clicking the TestRail login button below the standard login.

If you have any questions or need assistance. Please email us at support@assembla.com