It is not very clear.
Could you post a sketch showing:
what are the options (custom and another)? Are there 2 buttons ? Or only one button which behaviour should change depending on the slider value ?
which another option is it ?
Is the TextField inside TableView row ?
when the custom option is tapped, bring up a row immediately below there and have a UITextField
You bring up a new row ? Or an existing one ?
do you add a TextField, or was it already in the row ?
When another option, let's say 10%, is tapped, I want the text field row to go away.
Do you want to delete the row ? Or just hide it ?
If there is only one button which behaviour change depending on the slider value:
in the button action, you test the value of slider
if 20% or more, then call mytableView.insertRows(at…)
if less, then call mytableView.deleteRows(at…)
In both cases, don't forget to update the dataSource.