Unless you have a separate, 64-bit compatible version of mscomm32.ocx, you're going to be restricted to using the version you have in a 32-bit application, because that's the architecture the ocx was created for.
Look at sysmon.ocx, for example. On my Win10 machine, the 32-bit version of this ocx resides in the C:/Windows/SysWOW64 folder and it's size is 337,408 bytes. The 64-bit version resides in the C:/Windows/System32 folder and its size is 423,936 bytes. Two versions for the two architectures.
I suspect there might not be a 64-bit version of this particular ocx, so you'll probably have to find a different means of accessing COM ports in a 64-bit application.
mscomm32.ocx works fine with Windows 10 (64 bits)
The problem is when using mscomm32.ocx with a powerbuilder application compiled in 64 bits.
I'm looking for a new version of mscomm which is compatible with 64 bits applications and that we can integrate in our powerbuilder development as an ocx.