How to setup Clickatell
Here's a link to the current docs for setup of CiviCRM / Clickatell.

In that there's an example that suggests api_key=XXX is the correct format. It gives this example (when using a specific 2-way config):

api_id=1234567
from=15558238168
mo=1


Those docs also give instructions to set up an API product and obtain the key (which it looks like you've done), and say "Be aware that your Clickatell API password may differ from your Clickatell account password." which may be the problem you're hitting?

If you're getting a meaningful "authorization failed" response back I'd be inclined to take that as it is given and verify those authorization details (username, API password and API key) are configured right.

Next steps might be to debug around this line to see if the parameters are being set correctly, or to seek out support from someone able to do so.