Use this:

<OutputClaim ClaimTypeReferenceId="Flag1" DefaultValue="{OAUTH-KV:Flag1}" AlwaysUseDefaultValue="true" />

where Flag1 is the value in the query string.

Then do:

<OrchestrationStep Order="x" Type="ClaimsExchange">

<Preconditions>

<Precondition Type="ClaimsExist" ExecuteActionsIf="false">

<Value>Flag1</Value>

<Action>SkipThisOrchestrationStep</Action>

</Precondition>

</Preconditions>

<ClaimsExchanges>

<ClaimsExchange Id="Call-API" TechnicalProfileReferenceId="REST API" />

</ClaimsExchanges>

</OrchestrationStep>

and similarly Flag2 would call another API