Will a 45W charger work with my 2018 MacBook Pro 16-inch?

For reference, there is no 2018 16-in. MacBook Pro model. Did you mean the 2019 model (MacBookPro16,1 or MacBookPro16,4) model?

Regardless, these models came with a 96W USB-C power adapter & 2m USB-C charging cable. The other comments, provided by the others, still applies.

It will work, but it may not have enough power to charge the battery while actively using your Mac. That means it may still draw power from the battery, but it will discharge more slowly than it would without a power adapter.

In addition to what @John Galt mentioned, when the laptop is powered off, then this small power adapter will slowly charge the laptop.