Hello we have a textbox in which when user click on that textbox it should

1. If you working on Versions earlier to 7.4 then onFocus of textbox autosubmit the form and call the query rule outside the form then loop it back to the same UI and populate the value.
2. If it is 7.4/7.5 then on saveInto of other field in SAIL you can call the query rule to set the DB column data.
Ex: local!textValue << rule!APN_returnFirstInput(rule!QueryRule,_), Set this local!textValue as a default value to textbox.

You can use a!pickerFieldCustom component to accomplish your requirement
Please refer the link for more on picker fields
forum.appian.com/.../SAIL_Components