It's "83(b) election." It's interesting a late stage startup is offering this. But do NOT do it.

Do this only for very early stage startups where your total expense might be a few thousand dollars for maybe 1% or more of the startup. At most, you lose a few thousand dollars.

Spending $100K to optimize possible future taxes is a bad idea. At this stage, you should just think that future AMT is a good problem to have. Put the $100K in safer instruments.