Adding fields to any form in Senta is very straightforward. First, go to Settings > Forms
You'll see a list of all the forms on your Senta site. Click on the form you want to edit. In this example, we're changing the Edit client form, which is where the main client details are, but you can edit any form you like. You'll see the form's name at the top -- and then a list all the tabs (most forms have just one tab called "General").
Click on the appropriate tab. You'll see all the fields that are on that tab.
Click on the Add field button to add a new field. You'll be prompted for various bits of information.
One box that might warrant some explanation: the internal field name. Here's a bit of information about it on our support knowledge base.
TL;DR: Add a name like mynewfield or clientcode