There are several options here:
You can initially accept as a partner a person with the relevant knowledge and experience. If you have this opportunity then your startup is lucky. It is the cheapest case but this is unrealistic.
You can get an appropriate specialist - freelancers. This situation is similar to buying a pig in a poke, however. You can easily become a victim of a banal scam. Some swindler with a “well-spoken tongue” could well make a hole in the finances of a young company, at least a couple of tens of thousands dollars.
The Product Engineer job is highly valued one by potentional employers. Their salaries start at $2,000 (yes, in real life engineering and scientific concepts cost a lot). You can find such employee in a product engineering services that specialize in providing such highly qualified employees to the companies in need, finally.
The last solution is definitly the best because you will be able to safe a lot of resources (time first of all) in that case.