To open/show the form with with your own button or link you can use:  srv.openModal(true,'formkey') 

This javascript function can be added as onclick event, for example. 
 You can add this code to your button: <a onclick="srv.openModal(true,'form-key')">feedback</a>
 
You need to specify it with the form key, which is the unique identifier for that particular feedback form. You can grab that key from the preview page:
https://app.mopinion.com/survey/manage/preview/2a8f783514fa91e03666bbb8c444ac4a0b967074
 
The full code for a link or button would then be:
<a onclick="srv.openModal(true,'2a8f783514fa91e03666bbb8c444ac4a0b967074')">Your feedback</a>
 
To check if it works you can also run the command from browser console (right click: Inspect element, and enter the javascript function without the html tags, so remove <a> and onclick event). Like this:
srv.openModal(true,'2a8f783514fa91e03666bbb8c444ac4a0b967074')
 
Then additionally, to hide the Mopinion button, go to the feedback form editor. (Replace "app" for your dedicated Mopinion subdomain if needed.)
 
And then click the Design Tab, and then the custom css icon in your theme. 
 
 
There you add this line, but be sure to replace survey-key with the identifier of your particular feedback form:
 
button.btn-open-survey[data-button-for='survey-key']{display:none;}
 
And that's it... You are good to go!