Thanks for you answer.
I did some tests to linked Assembla and TestRail:
1- In TestRail, I create the link from administration > integration.
But I can lined only one space assembla for all projects on testrail.
2- In Assembla, I create a link from integration with the desired project in TestRail.
I found the TestRail tab in Assembla tickets. But in the TestRail project, I found the assembla space linked in the point 1).
If I delete the link from TestRail, I can’t anymore found the assembla space.
Finaly, I manage to link each Assembla space to each TestRail project but in TestRail I didn’t found the link.
Sorry if it’s not very clear ^^’