I created a Jewish app, and I was wondering if I can post a link to the app on Mi Yodeya, would that be considered Spam?
If you believe, for example, that the existence of your app is relevant to answering a question on Mi Yodeya, it may be OK to write an answer to that effect, but make sure to read this page from our Help Center first: https://judaism.stackexchange.com/help/promotion.
You can do so in a "community ad". Unfortunately, Stack Exchange is not currently accepting such ads. Keep an eye out here on Meta, or keep an eye on this Meta SE post, to know when it will do so again.
There's also a "Web site URL" (or some such) field in your profile, and you can link to your app there.