Regarding the JS edit, I assume that you read the following document?
Kintone Developer Program - JSEdit
Precisely what part are you having difficulty? Is it a kintone specific stuff?
And about the concatenating stuff that you are trying to do, you could do this by the native feature.
Try to do the following:
1) Place three fields: the dropdown field, the equipment ID number (text) field, and another text field
2) In the 2nd text field settings, place a check on "Calculate automatically."
3) After you place the check, you should see a blank space right beneath it, enter the following:
"Field Code for the Dropdown Field" & "(Blank Space)" & "Field Code for the Equipment ID Number field."
Kintone Help - & Operator: Concatenating Strings
By doing the above, it should concatenate the texts from the selected fields even after you make them as a table.