richTextItem, safeLink issue

I believe you need the full URL e.g. for your example this needs to be: https://www.google.co.uk/.

You'll need to examine the value entered by a user and validate it (which you should be doing anyway) and perhaps add the https:// prefix if they haven't already provided it. Or you could simply insist that a fully qualified url is provided.

How to insist that a fully qualified url entered by the user. Is there a way to use any kind of function to do this in Appian.

concat("https://", local!yourUrl)

But before you do this, make sure to check if the URL already consists of this. You can do this using like() function. 

I have to find whether user entered "http://" or "https://" then in this case also, I can use like() function?

You will need to verify it one way or another - there are numerous Appian text functions which you could use to do this, depending on how strong the need will be - you could use like(), left(), regexMatch, etc.

To simplify the data entry of a fully qualified url, I would simply replace any existing occurrences with an empty string and then normalize it at the end. You should also validate it to make sure it is a valid URL, I would suggest using a regular expression for this. https://community.appian.com/b/appmarket/posts/regular-expression-functions.

a!localVariables(
  local!value: "https://google.com",
  concat(
    "https://",
    reduce(
      fn!substitute,
      local!value,
      merge({ "http://", "https://" }, { "", "" })
    )
  )
)

To simplify the data entry of a fully qualified url, I would simply replace any existing occurrences with an empty string and then normalize it at the end. You should also validate it to make sure it is a valid URL, I would suggest using a regular expression for this. https://community.appian.com/b/appmarket/posts/regular-expression-functions.

a!localVariables(
  local!value: "https://google.com",
  concat(
    "https://",
    reduce(
      fn!substitute,
      local!value,
      merge({ "http://", "https://" }, { "", "" })
    )
  )
)

Thank you all for your suggestions. Really appreciate.