What you are looking for is a compound literal. This was added to the language in C99.
Your first case:
struct MyStruct q = {x,y,..,z};
is a syntax specific to initialization. Your second case, in the pedantics of the language is not initialization, but assignment. The right hand side of the assignment has to be a struct of the correct type. Prior to C99 there was no syntax in the language to write a struct literal, which is what you are trying to do. {x,y,..,z} looked like a block with an expression inside. If one were inspired to try to think of it as a literal value, though the language didn’t, one couldn’t be sure of its type. (In your context, you could make a good guess.)
To allow this and resolve the type issue, C99 added syntax so you could write:
q = (struct MyStruct){x,y,...,z};