Let me see if I have this correct @james_aan - you're looking to plug a mic, with 3.5mm jack-plug into a 1/4" ( 6.35mm ) jack-socket ?

I do that regularly with my FX box and use a simple converter like this - it costs pennies (well, not a lot! ) 

There are stereo (as you can tell this one is) and mono versions, and your 3.5mm mic plug will (almost certainly) be mono - BUT - it won't matter.

Can you separately test the mic ?   Like plug it directly into a 3.5mm input socket on your laptop or something and try to record or just route it to the laptop speakers ?

Since you say the mic uses a 3.5mm plug - it is *unlikely* that the mic relies on phantom-power ( it would almost certainly be XLR  if it did) - but if it is some strange kind of mic needing phantom power - then - it simply wouldn't work from the "guitar" inputs as they would not be providing the phantom power.    As I say - this is a most unlikely scenario though, PP is usually a 48V supply, selectively fed through the XLR (or generated externally by some kind of stand-alone box ) to devices that require phantom.