From now on, I identify myself as Black and everyone else should acknowledge that that's how I feel inside, regardless of my outer appearance.
My preferred race identity is African American.
Needless to say, I now qualify for affirmative action when that is available.
If you don't call me Black, you are a racist

Not if it's not true.

I mean, there are lots of thorny issues regarding self-identification (especially on racial matters) that even progressives disagree on, but no one thinks that a pretextual claim of self-identity has to be honored. You can't just say the phrase, "I identify as X" - and nothing else - and have it be true. It's not a magic incantation or ritualistic expression that becomes truth merely because you say it.

Certainly that's true in most legal contexts. For example, a person who has never engaged in any observation, study, or practice of the Jewish faith can't just declare one Friday afternoon that they are an devout Jew and require their employer to accommodate their need to observe the Sabbath. The courts can and will interrogate the sincerity of one's professed religious belief - whether you actually believe what you claim to believe.

But it's going to be true in most social and institutional contexts as well. People who have falsely claimed to be a particular race or ethnicity end up getting savaged - even by progressive groups. Rachel Dolezal is perhaps the most notorious example, but others (like Jessica Krug) have also had severe reputational and career damage for pretextually claiming an identity that they did not in fact have:

theguardian.com - Jessica krug white professor pretended black

You can't just decide one day to be Black.