First make sure you have identifiers for MacOS Application signing - grab that certificate from developer.apple.com
See your valid identities, which should list something like "Developer ID Application: Your Developer Name (10 Digit Code)"
security find-identity
Code Sign it
codesign -f --deep -s "Developer ID Application: Your Developer Name (10 Digit Code)" AppName.app
Check it
codesign -dv --verbose=4 AppName.app