DeclareFunctionMBS with a structure parameter?

I am currently trying to debug a third party API which works ok on macOS & Windows 32 bit but fails in one call on Windows 64. Natively, this function expects a ptr to a device and a structure consisting of two Single floats. It returns an Unsigned Integer. When I try to set it up with Var f As New DeclareFunctionMBS("(pT)J", p), I get an exception telling me T has a wrong calling convention. Which I don’t understand. Isn’t standard system calling convention ok here?
Any hints on how to set this up?
Thanks a lot!

Edit: Sorry, incorrectly copied.
The exception message is Invalid calling convention signature: T
Isn’t T supposed to stand for a structure? Does it have to be defined more in depth, like maybe T{ff}?