RCA to Optical Converter

No guarantees 

https://www.amazon.com/Digital-Converter-Hdiwousp-Aluminum-Headphone/dp/B0863F49PS/ref=mp_s_a_1_2_sspa?adgrpid=174874538125&dib=eyJ2IjoiMSJ9.x0RYVSUNcVpInxANAaY0bdR1GHfCrxzbmipq0zoMR7OJV4gVzuTpbbcES7_CIXmvUeabThFMbY3NFmx_HL1jNKuVAbx03rstLTXxwnfYH7vAocAttenHqU4NBBXCPQzKT-2U8iyJh7Z6opqWEo6Ae-yLkQoazdQTbBnmBkha9qSEBtjcbUqPbK7xYJSG-DXm5BOhzBNksEJtQ-u36dR9Pw._oeoZrrVBlsBveQvAC_YSRH7UomHoZvWsVJQsCw6F-c&dib_tag=se&hvadid=739151557381&hvdev=m&hvexpln=68&hvlocphy=9033780&hvnetw=g&hvocijid=4755859377969492446--&hvqmt=e&hvrand=4755859377969492446&hvtargid=kwd-306150123533&hydadcr=16190_13526812&keywords=analog+rca+to+digital+optical+audio&mcid=b4b5ba811ec534e4a15c85c3a9ec90bd&qid=1753828347&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRm&psc=1

@jkampmeyer 

Also, you would need to use the "Pre Out" L/R outputs into the Digital converter that @gkelly posted. 

And, with not completely understanding what you are trying to accomplish, remember, it will only be a stereo signal into the soundbar, so if you will not get any kind of surround sound.

@jkampmeyer I can confirm I have used a solution like @gkelly suggested, going from RCA to optical for a wireless headphone system, and it did work. Quality was within expectations for such a setup.

My challenge is that each of the speaker outputs has a red & white RCA female jack (a total of 4) I cannot find a DAC that has 4 RCA inputs. Also, I don't think I need a preamp, as my receiver has a phono input.

Confusion here are we talking about the pre-out on the back of the receiver? It appears that there are two, 1 for the left and right front Speakers. The other ones are for zone two set of speakers. Use the one for the left and right front.  Ignore the zone two one for now. 
also, another alternative would be to lose the Bose Sound bar and just get a set of speakers. Can guarantee this will have a better sound outcome than what you’re attempting to do. Look on craigslist or Facebook marketplace. 
Another question why are we not just hooking it up straight to the TV?

It is not a pre-out.  It is the + and - for the left and right speaker.  Instead of using a bare wire terminal for each connection, they used RCA jacks.

So you need an ADC instead of a DAC.  An analog to digital converter.  Here's one. 

@erik_squires 

+1

Better, a complete rethink of what the OP is trying to achieve.  Had to check that it is not April 1

@jkampmeyer 

Something is not making sense here.  Here is a link to a Onkyo TX-8050 receiver currently on eBay.  Is this what you have?

https://www.ebay.com/itm/406013771468?_skw=Onkyo+TX-8050&itmmeta=01K1ENMJTDYR410M8VBE5NXHTB&hash=item5e884e7ecc:g:n6YAAeSwFWNoYsIr&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1dBZCneLj95Ml8n5aqNVNIxLqd3k3THPrvQLztbKdZ301s6kTb%2FleXmAXiYPCOynk1NneRXysap%2BhrK58mDAGv%2FNJ8h3qgSc2PE9SJr1kmAvIOvzOAubVrbnLARheAdPH1YspNul09OogyLsvw%2FJTWJIJWh3%2BEmoUOEeIpB8yQWt66Td0aG68Rvt1TT5wGhpEKc7VF3qiSgqfjuPDj3gBA%2FgoxMioKSNQFuL00sLJNnCJR7DXpUGyP138dq73HQNfr9sZHJdIH8v0zocuyVmR%2FecUYjd0ukS3%2BAiVPF4WT3xg%3D%3D%7Ctkp%3ABk9SR86t0tWLZg

These are speaker level outputs from the receiver and the soundbar needs a digital output in optical form. If I have described this correctly above I would start anew. This seems difficult to achieve and many better options should be possible like ditching the soundbar with its limitations.

@Jbs - Oh good catch, for some reason I thought these were preamp outs. 

OP:  If this is coming from a modern TV, why not go from the TV to the sound bar directly? 

@jbs and @erik_squires

Based on what I see on the back of the receiver and according to the manual, there are a set of preamp outputs as well as a set of preamp outputs for a second zone.

@jkampmeyer - I'm going to ask a dumb question, so please don't take it the wrong way...  Are you referring to the big round red and black +/- speaker connections as RCA connections?

Legit question, why would you use a receiver with a soundbar? What are you trying to achieve? Best use of a soundbar is HDMI from the TV to soundbar. I just can't understand why you would want to us an amp to another amp (powered soundbar)