In that case it really depends on your application. There are many ways of sending custom message properties with IBM MQ Native mode, and you have to know which way your application expects.
The most common way is by using an 'RFH2' header with a 'usr' folder. In the 'Contents' area of the IBM MQ Native Send Receive step, click the gear icon on the upper right. Select 'IBM MQ Native Message' -> 'IBM RFH2 Header':
Under 'RFH2 Folders', click '+' -> 'Basic' -> 'usr':
Click 'Open Editor...':
Click '+' to add a property. There are a number of possible data types available, the most common one is 'String':
Enter your property name and value:
One last thing. If you have to also *receive* messages that have custom RFH2 properties then it's possible you have to make an adjustment to the 'IBM MQ GET' operation. Some versions of MQ require this, other don't.
Find 'Get Options' and click 'Open Editor':
Find 'Property Options' and change it's value to 'Force MQRFH2'. If your version of MQ is one of the ones that require this setting, then without it you will just receive a plain message with no embedded RFH2 header.
Again, this is just the most common way. There are other RFH2 folders you can use for sending custom properties, and other ways of doing it that don't involve RFH2 at all. It depends on exactly what your application is expecting. If your application was using JMS then we would know exactly what it expects. But since it's not, I can only tell you the most common way in the hopes that it at least gets you close.