PREFIX ex: <http://a.example>
PREFIX sx: <http://www.w3.org/ns/shex#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

[] a sx:Schema ;
    sx:shapes (_:S1) .
_:S1 a sx:ShapeDecl ; sx:shapeExpr [ a sx:Shape ;
            sx:expression [ a sx:TripleConstraint ;
                    sx:predicate <http://a.example/p1> ] ] .