If you list the marital status in your profile as either married, engaged or living with your partner, you will be given the option to add their details to your 1form account after you complete your profile and press "Save & Next" at the bottom of the page. This creates a joint account.
Alternatively, your partner needs to create their own account. You can then link them to your application by adding their email under the "Occupancy" section when completing your online application.
View Adding applicants to your application for further information.