AMP Superseal ECU Connectors with backshell

Just recieved my AMP Superseal ECU connectors 26 pin and 34 pin with the apropiate backshell for adding a heat molded boot. This backshell only slides into place? I doesnt seem this is a reliable method and Im sure Im missing something or Im not aware of the apropiate procedure to secure the backshell permanently to the connector. Can someone share how to do this since I want a completely sealed harness and this will not be sealed! Thanks!

Here are some pictures of the connector and backshell.

For a truly sealed harness you will have to epoxy them on. If seal isn't critical they are generally tight enough to just "press fit".