An ISWC stands for International Standard Work Code.
ISWCs are a unique identifier that your PRO (e.g., ASCAP, BMI, or SESAC) assigns to your Song once it has been registered with it. If you have never registered the song with your PRO, then you do not have an ISWC for the Song. Once you've registered the Song through your TuneRegistry account, you will receive an ISWC that will automatically be added to the Song's details.
If you have already registered a Song with your PRO, then you can get the ISWC from your PRO and add it to the Song details page in your TuneRegistry account.
To find an ISWC for ASCAP or BMI, you can search each of their public repertoires. For SESAC, in general, you can find the ISWC in your online account or by contacting your representative.