I want to know for each of my contacts if the already have a Threema secure message account to contact them.

Links to API documentation below

Search ID via phone number
https://gateway.threema.ch/en/developer/api#:~:text=Find%20ID%20by%20phone%20number&text=The%20phone%20number%20must%20be,as%20a%20text%2Fplain%20response

Search ID via email
https://gateway.threema.ch/en/developer/api#:~:text=server%20error%20occurs)-,Find%20ID%20by%20email%20address,-URL%3A%C2%A0https