As already said, 62304 is a standard that "legally" applies to the manufacturer of the actual medical device. Also there are many decisions that can only be made from the point of view of the medical device (e.g. software classification, risk management etc).
However, in your situation it makes sense to use certain assumptions and try to prepare for 62304 as much as possible.
Then you can make a document which declares these assumptions and also says which parts of 62304 you meet. You can give this document to your clients when they ask about 62304.
Be careful not to claim that you "comply with IEC 62304", only the manufacturer of the medical device can do this.