Go to My Account > Settings > Edit Organization (in menu button in top right corner) in your Txt2Give dashboard.
Scroll down to ChMS settings and select Seraphim
Enter your Seraphim Community ID (You'll need to contact Seraphim to get your Community ID)
Click Update Organization
Link your Causes
Select Giving > Causes > Edit. Check the first location box that you have set up (Main), and choose the Seraphim category that you want the donation to go in to. Click Update Cause. Repeat for each location that you have set up.