Hide menubar when using BUTTON with parameter target

I would like to use BUTTON to link back to the start page with a hidden menubar. When using a common link I would simply append ?cover=plain to the link. How can I achieve a hidden menubar with a BUTTON parameter without using href? The problem with href is, that foswiki is embeded within a frame and the foswiki in the url in the address bar remains the same (the foswiki start page) even if I navigate through the wiki articles. This also means, that BUTTON in combination with href has no effect and I remain on the site I am looking at. However, the parameter target works perfectly fine but I wasn't able to hide the menubar and I don't want it to be seen. Or what would be the best way to solve this? This is the code I'm using:

%BUTTON{
    "%MAKETEXT{"StartPage"}%"
    target="Main.CustomStartPage"
  }% %CLEAR%

If I change target to target="Main.CustomStartPage?cover=plain" it gets redirected to CustomStartPage %3fcover%3dplain which leads to a "Missing or illegal web name"-error.

I'm using foswiki 2.1.6

-- CaroGroote - 02 May 2018

I managed to get this working. I accidently had an error in the href parameter. It worked by using:

%BUTTON{
    "%MAKETEXT{"StartPage"}%"
    href="%DEFAULTURLHOST%%SCRIPTURLPATH{"view"}%/%USERSWEB%/CustomStartPage?cover=plain"
  }% %CLEAR%

-- CaroGroote - 07 May 2018

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