You can do all the recording ok into the Tascam. But to use those sounds in the looper in real time, you would need to connect an out from the Tascam to an input on the looper.
So it depends what inputs are available.
I have a Tascam but not the Boss so only know half the equation, but below are observation which may assist.
The Tascam only has three potential outputs.
1 : The usb c port, Does the Boss have a USB input port?
2: the headphone out port, (it’s possible but you would have to use the Tascam headphone volume control to ensure signal balance and not overload the looper input. You also have a cabling issue with headphone out being a stereo out on a TRS jack and the Boss input possibly being Mono TS Jack. The headphone out will have Left and right audio signals, one on its ring and one on its tip. If the Boss in is stereo on a single TRS input you should be okay but I suspect it’s Mono.
3 : The Ext camera out port. This is a line level output on a TRS Jack so again possibly stereo to Mono cabling issues.
Via the correct cabling you should be able to link the devices but a lot depends on how or if the Boss can use line level at its inputs.
Hope these observations assist.
Cheers
David