Inclusion Criteria: - Ambulatory participants with stable HFrEF due to any etiology. - Body mass index (BMI) of 18.0 kilogram per square meter (kg/m2) to 35.0 kg/m2 inclusive. - Documented left ventricular ejection fraction (LVEF) 15% to 45% (on 2 occasions), including at least once during Screening and confirmed by the Echo Core Laboratory (the absolute difference between the 2 LVEF values qualifying the participant should be < 12%). - Participant receiving chronic medication for the treatment of heart failure reflecting current guidelines, including at least one of the following, unless not tolerated or contraindicated:β-blocker, angiotensin converting enzyme inhibitor, angiotensin receptor blocker, or angiotensin receptor neprilysin inhibitor. Such treatments should have been given at stable doses for at least ≥ 2 weeks prior to screening with no plan to modify treatments during the study. - Sinus rhythm or stable atrial or ventricular pacing or persistent atrial fibrillation that is adequately rate-controlled to allow pharmacodynamic (PD) assessments by Transthoracic echocardiogram (TTE). NOTE: Participants with implanted cardioverter defibrillator (ICD), pacing, or cardiac resynchronization therapy are eligible provided device programming is unchanged starting 2 months prior to and throughout the dosing period. - Adequate acoustic windows, determined by the Echo Core Laboratory, to enable accurate TTE assessments. Exclusion Criteria: - Presence of disqualifying cardiac rhythms that would preclude echocardiographic assessments, as determined by the Investigator, including: (a) rapid, inadequately rate controlled atrial fibrillation or (b) frequent premature ventricular contractions that might interfere with reliable echocardiographic measurements of left ventricular function. - History of bronchospasm, or history of respiratory depression or arrest, airway obstruction, oxygen desaturation, or apnea. - History of allergy to midazolam, other benzodiazepines, danicamtiv, related compounds, or excipients in the formulations. - Severe renal insufficiency (defined as current estimated glomerular filtration rate [eGFR] < 30 mL/min/1.73 m2 by simplified Modification of Diet in Renal Disease equation [sMDRD].