My understanding is that the Message Type determines the Format. For instance, I believe MQSTR is set automatically if the IBM MQ Connector bridges a Solace TextMessage to an IBM MQ JMSTextMessage.
However, if you want to ensure it is set regardless of Message Type, you should be able to set the JMS_IBM_Format header using the Header Transform feature as follows:
[ … ]
solace:
connector:
workflows:
0:
enabled: true
transform-headers:
expressions:
JMS_IBM_Format: "'MQSTR'"
[ … ]