We previous began discussing what to do with tilnavn in the name fields of Wikitree in this G2G post. That discussion revealed some problems with the idea of putting tilnavn in the nickname field.
Since then, there have been additional G2G posts seeking help with various Danish naming questions.
See How do I handle Danish 2 surnames? and Danish Naming help.
Working through these 2 and working again on the profile of my husband's ancestor Niels Martinus Jensen got me to thinking and working on a slightly different proposal for tilnavn. Look at what I've come up with below and then at Niels Martinus Jensen's profile where I've applied it. There are two answers below to vote on this proposal. If you have comments please additional answers where we can discuss your thoughts.
Tilnavn Proposed Guideline
1. Tilnavn should be placed in the Other Last Name field without parentheses.
2. Since parentheses are not permitted in name fields and none of the name fields are a good fit for tilnavn, a name note or section should be included at the top of the biography section of all profiles with Danish names, especially those with tilnavn. The name section should first show the name as it would best be displayed taking into account all given names, true middle names, patronyms, slægtsnavn, and tilnavn used on known records. The tilnavn here should be enclosed in parentheses at the end of the full name as was often done in Danish church records. This name section may include a reference or link to the free space page for Danish Naming Conventions and Related Wikitree Name Field Guidelines. If differing forms of the name were used on different records, which names were used when should be explained in either this section or later in the biography as each record is discussed.