Doi can be submitted in two ways. First is send an email to submissions.authors@library.caltech.edu or click on the Add a DOI link in the navigation above.
In the latter case you will be presented with a web form containing two elements, the DOI field and a URL to the digital object if known. Full those in and click “submit”. If successful this will take you to the list of DOI page where you can then take the next steps.