@grdhog said:
Where do find the "sign up link" for the developer API please?
I cannot find the link mentioned in the below sentence on this page: https://www.themoviedb.org/documentation/api :
You can apply for an API key by clicking the "API" link from the left hand sidebar within your account settings page.

Here:
https://www.themoviedb.org/settings/account
https://www.themoviedb.org/settings/api