If secondary insurance is Medicare then you need to update "Insurance type code" in billing customization.
Following are steps to update "Insurance type code".
Search for the patient and then goto patient dashboard. Click on Financial information from related task.
Click on the information icon to view description for each code. Select the applicable code.
Navigate to Billing >> Insurance billing
Select "Secondary" in Bill drop down. Enter filters to generate insurance billing and then click Print "HCFA 1500"
Click "Download EDI-837P" to generate 837 professional claim.
Generated 837 professional claim.