Also, if the IHS surcharge is required, it will prompt you to pay it when you apply online.
As you are applying for a fiance visa, there is no surcharge to pay, and therefore you will not be prompted to pay it. If it tells you to pay it, that means you have selected the wrong visa type and will need to go back and start again.
For a fiance visa, you select: Settlement -> Settlement -> Marriage as the visa type
If it was a spousal visa, you would select: Settlement -> Settlement -> Wife (or Husband)