I am doing an MA and am preparing essay research about Funnyhouse of a Negro. I need help forming a proposal about the cultural work that Kennedy's play was doing in the sociohistorical context of her writing in relation to legacies of slavery and civil rights in the US.

When creating a proposal for your MA, it would be best to begin with creating a strong thesis. This particular thesis will be more broad in nature than the thesis for a regular paper. Next, you should create a rough outline from your thesis. I use an "ABC 123" method. The ABCs are your major points.
For you, A might be looking at what exactly you mean by sociohistorical context. Define that term so that you can then later apply it to the work. You may also spend time here on the events you mentioned.
Your 1, 2, and 3 here would be looking at each section of these closely. (This would be the same for sections B and C.)
B might be a look at the author: an overview of the author's life, how the events you mentioned have influenced the author's work, and so on.
C could be where you bring together the two (A and B). How does A present itself throughout the work? How does A weave throughout the play? What factors of B are present within the play?
Each of these sections may be rather long in nature, but if they are all working to support your thesis, then you are creating a solid paper.