You can make a form members-only, and show the Member Splash login in its place so members can sign in and fill it out without leaving the page.
Prefer to watch?
Make a form members-only
Go to Content > Forms, hover over the form, and click Settings.
Scroll to Restrictions and check Require user to be logged in.
Type the message visitors see if they aren't logged in, like "Please log in to register."
Check Show Member Splash Login so the login form appears under your message.
Click Save Settings.
Now visitors who aren't logged in see your message and the login form instead of the form:
💡 Using Member Splash fields? Fields like Member Select only work when the member is logged in, so turn this on for those forms.