FR165 - How to account for using a weighted vest for walking (or, for running or for strength training)?

No. But your HR will be increased due to more effort and you can write into workout Note the weight you used.

I’m new to Garmin and plan to log my runs as hikes when I have a weighted vest and/or jogging stroller.

Edit the activity in Garmin Connect, and add the weight there. It is available at runs too.

I don't see the "pack weight" option. Is that something i have to set? I added as "gear"

I do not see it anymore either. Looks like Garmin pulled it away. It probably just leaked out for a few days, while not fully implemented yet. Or they changed their mind. Unfortunately, I do not know more.

It is there back again, when you edit the activity. I checked Run, Trail Run, Rucking, Walk, but possibly it is at other activity types as well.

Thanks. I did add the weighted vest as GEAR so I could add it if I use it. I don't see pack weight anywhere on the app. Do you think it's just on the computer version?

Although I understand what you mean, GC Web is not a computer version. It is a web based app, and it works on phones, or on any other web enabled devices too. And yes, it looks that currently it is only available on the web version.

As @9062534 posted, I do not find "PACK WEIGHT" available. It doesn't appear in GC mobile app nor in the web based app. Also, I searched for "RUCKING" to add to my FR165 activities and could not find any activity that I could associate with a weighted vest.

I tried adding the weighted vest as GEAR; but, that seems to only act as an equipment reminder or, maybe, as an equipment tracker for mileage.