Here is a step by step guide (I think this should come pre-loaded with the add-on):
- In the top menu of RESTClient -> Headers -> Custom Header
- In the pop-up box, enter Name: Content-Type and Value: application/x-www-form-urlencoded
- Check the «Save to favourite» box and click Okay.
- Now you will see a «Headers» section with your newly added data.
- Then in the Body section, you can enter your data to post like: username=test&name=Firstname+Lastname
- Whenever you want to make a post request, from the Headers main menu, select the Content-Type:.. item that you added and it should work.