Only API connection of kind: 'V2' can return a connectionRuntimeUrl.
You can create a cosmos db connector with the below script (bicep):
param location string = resourceGroup().location
param cosmosDbAccountName string = 'thomastestcosmos'
param connectorName string = '${cosmosDbAccountName}-connector'
// get a reference to the cosmos db account
resource cosmosDbAccount 'Microsoft.DocumentDB/databaseAccounts@2021-06-15' existing = {
  name: cosmosDbAccountName
}
// create the related connection api
resource cosmosDbConnector 'Microsoft.Web/connections@2018-07-01-preview' = {
  name: connectorName
  location: location
  kind: 'V2'
  properties: {
    displayName: connectorName
    parameterValues: {
      databaseAccount: cosmosDbAccount.name
      accessKey: cosmosDbAccount.listKeys().primaryMasterKey
    }
    api: {
      id: subscriptionResourceId('Microsoft.Web/locations/managedApis', location, 'documentdb') 
    }
  }
}
output connectionRuntimeUrl string = cosmosDbConnector.properties.connectionRuntimeUrl
The url will be an output of the generated ARM
You can then set this url as an appsetting in the workflow app:
COSMOS_CONNECTION_RUNTIMEURL: <connectionRuntimeUrl>
Then in the connections.json file, you can reference this app setting:
{
  "managedApiConnections": {
    "documentdb": {
      ...
      "connectionRuntimeUrl": "@appsetting('COSMOS_CONNECTION_RUNTIMEURL')"
    }
  }
}
Using appsettings and parameters should make thing easier to deploy