How is your itemtype field analysed?
If it is of type string , then use:
fq=itemtype:"Supply Chain Intermediaries"
Otherwise you can also try:
fq=itemtype:(Supply Chain Intermediaries)
Assuming OR is the default operator in your config and text is the default search field, your query will get translated to:
fq=itemtype:Supply OR text:(Chain Intermediaries)
Chain and Intermediaries are searched against default search field.