Go to https://giphy.com/ and sign up or login.
Then click on 3 dots like image below & find Developers option, click on it.
This will lead you to https://developers.giphy.com/
Click on Dashboard.
Under your API Key section, click on Create an API key
Select API option and click on Next step
In this step,
1. Give name,
2. Select Platform,
3. Enter Description,
4. Click on Create API key.
And you will find the API key which you can use.
Find the button under your API key and finish the process.