How to capture a value inside a selectbox column, and concatenate?

I'm trying to build a system that creates a reference number, using an ID created when a new record is saved.. 

The user makes a series of selections using a dropdown lists in a view and then presses Submit. Based on the unique ID and the values Selected in the DropDownLists I want to make a ReferenceNumber consisting of The UniqueID from the Proposals Table and Initials that reside in the DropDownLists source Tables. For example:

 

ID

ProposalCategoryID

ProposalReference 

 

ProposalCategory.cs  

ProposalCategoryId

ProposalCategoryName

ProposalCategoryIntials

 

On the View Page the user makes a series of SelectBox choices and presses Submit. This calls an IACTION with end up with an HttpPost Action 

 

I have been able to save the new record, capture the new record ID and the ProposalCategoryId by Saving and capturing the ID into an int called: myId and a var called catId

 

I need to somehow get the ProposalCategoryIntials, I've tried so many methods but so far have failed. I did set up var PropoInitial = _unitOfWork.ProposalCategory.GetAll(); which when I put a breakpoint on I discover all the three fields from the Category database are in there. and all the field names are in there. All I need is the ProposalCategoryIntials based on the CategoryID that was posted. I can see it in the BreakPoint information but I cannot work out how to get it.

 Once I have it I want to add it to the string, I have ID and ProposalCategory in there so far:

 

 proposalVM.Proposal.ProposalReference = "Test New ID: " + "-" + myid + "-" + catid + "-" + PropoInitial;

 

I am using PropoInitial to try to capture the ProposalCategoryIntials, but as PropoInitial has three columns and all data it's causing an error.

 

Once I've built the string I then need to open up the details page send the ID which will then show the generated ProposalReference 

 

My issue is I can't get to  ProposalCategoryIntials and once I have I need to send an ID with the 

 

 return RedirectToAction(nameof(Details)); 

 

Here's the troublesome part of my ProposalController. 

 

 

 

 

The IUpsert  process the record is saved, at that point I can capture the unique Id of the new record.

I then want to build a string using that unique ID and values from the SelectListsand