hi,new person here.I am trying to add a text frame to a multileader text that has a custom box ie thick line with filleted corners. I went into the properties of the multileader and changed the setting on the text frame to ON. As per the normal autocad text frames they are just square boxes with the same line thickness and layer as the text they were created from. I want to have a thicker line with filleted corners. Anyone know what might work??

Welcome to the forums!
 
You kind of hinted at the solution with the phrase, "custom box".  I tried creating a custom user block with dynamic properties and filleted corners that could be stretched to fit text... but unfortunately the multileader style editor will not use dynamic properties in a custom block.
 
So you might be left with creating your own dynamic block that, for all intents and purposes, looks like a multileader but brings with it annotative as well as dynamic properties to do what you wish.  
 
Dave.