Head over to the Add fund page and use whatever currency you prefer to use for adding $2 into your balance.
Do note that cryptocurrencies give a bonus of 20% the first time you add funds and then 10% for each new recharge.
After adding funds, go to the API key link and copy your Account Key (never share it, it's confidential).
Edit your Invidious config.yml file (should be located in the config directory) and add your account key at the end of the file after the captcha_key parameter.
Here is an example: