https://help.quickbase.com/api-guide/add_record.htmlThat should be helpful. They have a part at the bottom showing how to use it in a URL.
Are your fields named "[[phone number]]" and "[[cutsom1]]", with the extra bracket?
You will need to URLEncode the fields you are trying to use in this URL.
If you look at one of the "Add" buttons that is created in a relationship it can help guide you in the right direction. Here is one cleaned up with the AddRecord function. If you are adding a record to a child table or related table then don't forget to bring along the record id for the "Related X" field on the child table so it knows what record it is related to in the parent. You may or may not need this.
URLRoot() & "db/" & [_DBID_MY_DBID] & "?a=API_AddRecord&_fid_1782=" & URLEncode ([Record ID#]) & "&_fid_7=" & URLEncode ([Phone Number]) & "&_fid_83=" & URLEncode ([custom1])-
I see you got it working though so nevermind, but URL encoding is important. You should do it or you may find yourself with some bad data or another failure depending what is contained in there.