How to create a parameter output like a slider does from a c++ OOT block?

I would like to compute the tune-to frequency rather than use a slider. How do I configure an OOT (written in c++) to have a parameter output like the QT GUI Range block?

You can't.

"Parameters" are a concept that doesn't exist in GNU Radio – it only exists in the Python flowgraphs that GRC generates.

Instead, you would simply have a block with a message port output, and on that output, you'd emit a message containing a dictionary that contains the instruction to tune to the right frequency. You'd then connect that message block to the message input of whatever Hardware-interfacing block (UHD USRP Source? Soapy Source?) you're using.