Welcome to the community, @Harris_Mirza! First off, let me clarify the terminology of Airtable’s system, just so we’re all talking about the same things.
When using a form field to select a record from another table, that field is a link field, not a lookup. A lookup is a separate field type that uses a link to pull related information about linked records.
Going back to your example, it’s not possible to use a link field in a form to add a record in a linked table, like your user table. You would need to add the new user record first, possibly using a separate form to do that, and then you could link to them in your main form. When using the Airtable system itself, you’re allowed to add new linked records on the fly if one doesn’t exist in the table you’re linking to. However, this isn’t possible using Airtable’s forms, primarily (guessing) as a safety/security measure. You wouldn’t want just anyone arbitrarily adding records, and the form has no way of knowing that you have an Airtable account, so impromptu record addition is not allowed in forms.