LastPass Form Fill provides many built-in fields. However, you may find yourself wanting to fill other fields as well. You can do so using Custom Fields.
When editing a Form Fill profile, click on the Custom Fields tab:
Each row corresponds to a single Custom Field. Every Custom Field must have its Text and Value boxes filled in.
Text to find contains the descriptive text next to the field you wish to fill on a website. In case there is no text on the website, it can also match the field’s name or ID. Matches are case-insensitive, and partial matches are allowed. For example, ‘MSN’ would match both ‘MSN’ and ‘msn id’.
Value to fill contains the value you wish to fill into the field. It can be multi-line, but keep in mind that multiple lines can only be filled into a multi-line text box on a website, not a single-line text box.