On the signup form on Floodgate, you are in 'Auto' address mode when only 1 text field is shown for the address. When you are in the 'Auto' mode, address suggestions are shown as you type in the address.
You can consider that all the addresses suggested by Floodgate are valid. It is always recommended to select an address from the suggestion.
When the address you want to add cannot be found in the suggestions, then you can add them manually from the 'Manual' address mode. Simply click the 'Manual Entry' link below the address line to get into the manual mode. However, please note that the address you added manually is not verified. There is always a chance that the form becomes not payable because the address is invalid.
Address field is shown in 'Auto' mode by default for most campaigns, but the campaigns in the Philippines are an exception. For the campaigns there, 'Manual' mode is displayed by default as the auto validation does not work properly. When you fill in the address in the Philippines manually, start typing the address from Suburb field, which will provide you the list of valid suburb in the Philippines. When you select 1 suburb from the list, other information such as post code and state are populated automatically.