I'll be paying income tax in the UK for what I earned before I went to CA. To avoid double taxation what do I put here when it asks for my income from foreign sources?

Because we have such a treaty with the UK, please follow instructions below to avoid being double taxed:

How do I enter foreign income that is subject to a tax treaty?

I have a similar question, but the link you've provided no longer works. Can you please provide another link so I can avoid the double taxation issue?