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You can define S directly as a field, by defining its modulus by hand (in case the point is to define it explicitely):

sage: R.<x> = PolynomialRing(GF(2), 'x')
sage: S.<y> = GF(2^8, modulus=x^8+x^4+x^3+x+1)                                                                                                                                                     
sage: S                                                                                                                                                                                                      
Finite Field in y of size 2^8
sage: (y+1).multiplicative_order()
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