Hi,
On datamarket.azure.com, you can click on my account, click on developer, and click on register. On register screen you can enter client id and client secret. If you do not have application, you can mention any valid or dummy url against
redirect URL and type any name against application name. Then You can use the client id, client secret and category (general if you do not have custom translator) in Microsoft document translator.
Here are some useful links:
https://hubtest.microsofttranslator-int.com/Help/Download/Microsoft%20Translator%20Hub%20User%20Guide.pdf
https://mix.office.com/watch/xu652a7t4avs
Let us know if you need any additional help.
Thank you