I want to jump out the mic so I can scan without a mic
It's been quite a while since I used the 1225 software, but I think that hook/scan is a software option and can be disabled by programming so that the jumper isn't needed. That was a requirement on the GM300's but not on newer radios.
But, if you have don't have access to programming, and it's set up for off hook disables scan, you need to jumper pin #3 (hook) to pin #4 (ground.) This is generally done with a RJ45 plug with a very small jumper inserted in the right spots and then crimped with a special crimping die. There are folks that make them up for sale and use a bit of clear caulk to seal up the backside of the plug.
If you don't have the plug or the die, there's two solutions, and they're both ready made. First, there are a bunch of sellers on Ebay offering
GM300 shorting plugs (use those search words and they come right up) but the downside is that it's going to cost you $6 for a single plug. Or $5 for a pack of 5 and then you will have 4 of them that you won't need.
The second choice is to get one from me. When I needed one, they only sold them in packs of 5. So I've got 4 of them here that I have no use for at all. I can send you one for $3 if you want. Shoot me a PM and I'll send you my paypal address.