You probably don't want to anyway. Direct import doesn't work well with investment accounts. None of mine are linked and I just update the value monthly.

Hey Tomato Cartridge ! Set it up as an Unlinked account. 😄 I like to handle Tracking Accounts like investments (or loans) by creating a Scheduled Transaction to show regular contributions or payments, then update the account balance in the YNAB web app like this monthly. Check out the links for a glimpse into the process Superbone  mentioned!