The first thing is to make sure you should not be using JMS instead. Most of the time, when RFH2 headers are involved it means MQ is being used through JMS and not Native. If you're sure you should be using the Native step, then RFH2 support is built into the 'Contents' section.
In the 'Contents' section, click 'Text' -> 'IBM MQ Native Message' -> IBM RFH2 HEADER':
Click the '+' button to add a usr folder, then click 'Open Editor':
Click '+' to add a usr property with the correct type, then you can enter a key and value:
If you need an RFH2 folder other than usr, then make sure the 'PRO' button is enabled in the upper left and you will have advanced options under the top-level '+' button: