How a member can enter text in field then update to display in Dropdown list

Good day,

I hope you are well?

I would like to find out how to allow text entered into text field as extra option in dropdown list then the member can click update so that the option can be instantly selected.

Please may I ask for your assistance or if you can kindly redirect me to the correct code?

I also have a few other questions regarding inserting a table look on a page then member can add values to be calculated and an average over time period can be worked out – Please can you direct me in the right direction for someone to assist with this.

Kind Regards,
Danie