It helps to lubricate the rubber seal.
My father in law (ex) ran a glass business for 50 years. He used ATF for this (yes, Auto Trans Fluid!).
As with many things, having some lubrication makes the whole process easier.
Once iot is in, you can wipe off the ATF and then apply the sealer under the lip of the seal.
When I did my 635 windshield, I put the trim in before installing the windshield and seal tot he car. It is pretty much impossible to get the trim in afterwards, although I'll bet the lube trick would aslso work there.
S