After you start working, after some time you'll receive a letter stating the price at which your shares will be allocated. It will also contain a link to Schwab registration.
Country of Residence on the first registration page means country of residence, so in our case, UK. Country of Citizenship means citizenship.
Then they'll ask for phone number, email, and home address (enter your UK address). They also immediately ask for Tax Identification Number (TIN) for countries where you're a tax resident. For the UK, this is the NI Number on the back of your BRP.
You can also install the phone app.
After registering with Schwab, letters start arriving asking you to fill out W-8*/W-9* forms.
If you're not a US citizen, you must fill out form W-8BEN, otherwise you'll have to pay an even larger tax than you already pay when receiving shares.
The trickiest part is specifying "country of citizenship for tax purposes." You probably only have citizenship of the country you came from, but you'll pay taxes in the UK. You need to specify "country of citizenship for tax purposes" according to your passport. If you specify UK, your form won't be accepted since you don't have a British passport.
An example of filling out the first part is shown below.

After filling out the first part, a message will appear saying your address is in the UK, not in your country of citizenship. Choose "None of the above" and write a simple explanation like: "I am a citizen of ____, but currently reside and pay taxes in the United Kingdom."
You may encounter the question "Is this submission being made to claim treaty benefits?" The US tax authority website has a list of countries with which the US has concluded double taxation treaties. The UK is on this list, and since you're a UK tax resident, you probably need to answer Yes.
Then they'll ask for a tax identification number (TIN) for the UK. This is the NI Number (NIN), usually on the back of your BRP. (It could also be UTR, found in the self-assessment request from HMRC, if you've already been in the country for more than a year.)
Carefully read what you're being asked in the form and answer truthfully. Don't follow the advice given here blindly, as your situation may differ.
Keep in mind that insider trading (and selling shares outside the trading window is exactly that) is a crime. If you do it incorrectly, there will be big problems.
To sell shares, you need to create an order — an instruction to sell under certain conditions.
In the top menu, select Trade and fill out the form. Specify:
An example is shown below.

Don't forget that if your annual gain (difference between the price at which shares were issued and sold) exceeds £12,300/year (the amount changes year to year), you need to pay Capital Gains Tax. On the other hand, if the gain is negative, you can request a tax deduction.
After selling shares, money goes to a currency account in Schwab. You can only withdraw it two days after the sale.
Two ways to withdraw to a UK bank:
In both cases, the transfer takes 2-3 days on average. For online transfers (first method), they may call to confirm.
Currently, there's no fee for withdrawing from Schwab and you don't need to contact support, so you can use this method by default, but see what's better for you. The second method might have lower fees.
You can open a US bank account through TransferWise, and withdraw, convert, or spend money through it since TransferWise issues a MasterCard.
Registration instructions:
Create a US bank account through Wise.
Register it in Schwab (Transfers & Payments → External Accounts → Link an External Account).
Registration takes a couple of days. Schwab will make two transactions of a few cents for verification; you'll need to use these amounts as a confirmation code.
Withdrawal instructions:

For trading shares, it makes sense to open a Stocks & Shares ISA since all income from it is tax-free. This significantly reduces headaches when filling out tax returns.
Note that there are different types of ISA and you can have several open simultaneously as long as they're different types.
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