Before you can add digital signature in PDF file, you need to get a digital ID, sometimes called a digital certificate. You can not only obtain a digital ID from a CA but also create a self-signed ditial ID with Foxit PDF Editor.
A CA is a trusted entity that issues electronic documents, called digital certificates, to verify a digital entity’s identity on the internet. A digital certificate contains a public key used to encrypt and decrypt your digital signature. Ordering certificates from Certificate Authority often costs you some money depending on the type of certificate you need.
Foxit PDF Editor for Mac doesn't support to create digital ID, it supports to add digital signature with existing digital IDs. In this case, you may create a self-signed digital ID in Foxit PDF Editor for Windows then transfer the digital ID to your Mac computer. You may refer to here for more info about how to get digital ID in Foxit PDF Editor for Windows. After having moved the digital ID to your Mac OS, you could double click on the digital ID file (.pfx file) to install it.
Or you may also choose to create Self-Signed Identity from your system's Keychain Access app.
To place a digital signature, you could go to Protect > Sign & Certify > Place Signature…, then drag a rectangle to insert the signature and save the document.