Well, if you’re capturing a person’s preferred time slot in a person field on the form, then you can just include the corresponding field token in the email to populate the time slot value in the field. {{lead.<preferred time slot field>}}.
Add a token to the email by using the Insert Token option in the email editor. Check out the tokens overview product documentation on the experience league.
Also, please be advised that tokens pull the value from the person record in the database, and not directly from the form post/submit. So, in a case where the field is updated between the filled out form activity and the Send Email flow step is run, Marketo will pull the updated value the person record has in the field, and not the one that was posted during the form submit. To prevent fields from getting updated from the un-wanted sources, For each field, Marketo allows users to block field updates from input sources.