Haven't written it, and haven't thought about it for about a month now, but probably something like:
"My name is [name], my next of kin is [name] and can be contacted at [address], [phone number], or [email address]. I have deliberately attempted suicide using sodium nitrite. The antidote is methylene blue. I would rather not be treated but understand that you have legal and professional obligations.
Sorry for putting you through this.
This wasn't an impulsive decision, but a cold, calculated one that was years in the making.
I believe I have a right to die and would like that right to be respected. I have consistently lied to friends, family, and especially medical professionals to stop them from sectioning me. My inability to talk freely about my suicidality has alienated me from those around me. I think the best way to prevent similar deaths would be legalisation of assisted suicide for anyone who wants it. Impulsive suicide is a public health issue, but determined suicide is a personal autonomy issue and conflating the two has made it harder for me to seek support."