You need to clean the metal groove that is right under the windshield with a toothbrush and soap, then fully flush it or wipe it clean.

In a similar fashion, you have to clean the gasket lip that catches in the groove channel under the glass.

To insert the gasket, you will need to apply some dishwash liquid to the groove, once you lay the rubber in the groove a gentle push will lock it in.

Hoping you have not smashed or broken that groove, you will then need to find it, I think its only sold in aftermarket now.

To insert the gasket, apply some dishwash soap to