Praise be to Allah.
It is haraam for a sick person to use anything that will lead to death, whether it is medicine or otherwise, and that is regarded as suicide, which is a major sin, because Allah, may He be exalted, says (interpretation of the meaning):
“O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent. And do not kill yourselves [or one another]. Indeed, Allah is to you ever Merciful.
And whoever does that in aggression and injustice - then We will drive him into a Fire. And that, for Allah, is [always] easy”
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Whoever throws himself down from a mountain and kills himself will be throwing himself down in the Fire of Hell for ever and ever. Whoever drinks poison and kills himself will have it in his hand, sipping it in the Fire of Hell for ever and ever. Whoever kills himself with a piece of iron will have that iron in his hand, thrusting it into his belly in the Fire of Hell for ever and ever.”
Narrated by al-Bukhaari (5442) and Muslim (109).
It was narrated by Thaabit ibn ad-Dahhaak (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever kills himself with something in this world will be punished with it on the Day of Resurrection.”
It was narrated that Jundub ibn ‘Abdillah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Among those who came before you there was a man who suffered an injury, and he panicked and grew impatient. He took a knife and cut his hand with it, and the bleeding did not stop until he died. Allah, may He be exalted, said: ‘My slave sought to hasten his death; I have forbidden Paradise to Him.’” Narrated by al-Bukhaari (3276) and Muslim (113).
It is haraam for a doctor or organisation to help a person to commit suicide, or call him to do that, or justify that to him, because that comes under the heading of helping in murder, but Allah, may He be exalted, says (interpretation of the meaning):
“And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty”
Based on that, what this organisation does of helping with euthanasia, directing sick people towards it and making leaflets and videos about it is a grave evil and serious crime.
It says in Fataawa al-Lajnah ad-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Iftaa’ (25/85): With regard to those doctors and others who respond to a sick person’s request for euthanasia and help him in that, they are sinning and their short-sighted approach is indicative of their ignorance, because they are looking at the individual’s life and survival in terms of his having physical strength, autonomy and desires; they do not look at his life in terms of his being connected to his Lord, striving to do more righteous deeds, showing humility and submission to Allah, and beseeching Him, thus becoming dearer and closer to Allah than one who commits acts of transgression and uses his physical strength in ways that incur the wrath of Allah.
Moreover, Allah, may He be glorified, is able to heal him; what appears in people’s minds today to be impossible may become easy to treat in the future, by the decree of Allah, for Whom nothing is impossible on earth or in heaven. End quote.
It is not permissible to work in promoting or marketing for this organisation, because of what that involves of promoting what it does of unlawful actions.
You said that you have completed the first part of your work and received payment for it. If this part of the work that you have finished may be used in marketing and promotion of this company, then your work has included both permissible and haraam elements. Therefore you must repent to Allah, may He be exalted, and get rid of part of this money, commensurate with the percentage of the organisation’s work that is unlawful. You can get rid of it by spending it in the public interest of the Muslims, or by giving it to the poor and needy.
Please see the answer to question no. 78289.
With regard to working for this organisation, if it is in a permissible field that has nothing to do with the haraam things that the organisation does, whether directly or by helping with them or promoting them, then there is nothing wrong with that.
But if your work involves helping with those haraam actions, or promoting them or supporting the organisation in all that it does, then it is not permissible for you to work there.
And Allah knows best.