I expect they just use this form for all PhD applications, whether applying for a particular project or submitting an independent proposal.
It says: "In up to 1,000 words, please provide a statement of your proposed area of research, and a short description of the study with details of the
aims and proposed methodology. Please indicate any relevant prior knowledge of the area and what you can bring to this piece of work. Tell
us why you believe you are the ideal candidate. Attach additional sheets as necessary."
If it was me, I would state what I'm applying for and confirm that the PhD already says should use xxx methods to achieve xxx goals. I would then just use the rest of the word limit to talk about prior knowledge and why I'm the best candidate. If you know/have experience with other methods then you could mention this too.
Good luck!